diff --git a/CLAUDE.md b/CLAUDE.md
index 0d5ba85..b2f044a 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -1,107 +1,227 @@
-# Reimbursement Form App
+# Reimbursement Form — Agent Reference
 
-Static browser app that collects expense data via a form UI and generates a PDF with attached receipts. No server, no build step — one HTML file + config.yml + assets.
+Static browser app that collects expense data via a form UI and generates a PDF with attached receipts. No server, no build step — one HTML file + config.yml + optional logo.
+
+## Repository layout
+
+```
+.
+├── CLAUDE.md               # This file
+├── README.md
+├── LICENSE
+├── docs/
+│   ├── user-guide.md       # End-user instructions
+│   ├── admin-guide.md      # Deployment and config.yml reference
+│   └── developer-guide.md  # Fork/extend instructions, design notes
+└── app/
+    ├── index.html          # Entire application (~870 lines)
+    ├── config.yml          # Runtime configuration (loaded via fetch at startup)
+    └── assets/
+        └── logo.png        # Optional logo (PNG or JPG)
+```
 
 ## Architecture
 
-```
-reimbursement/
-├── index.html      # Complete app (HTML + CSS + JS, ~850 lines)
-├── config.yml      # All configurable values (parsed at runtime via js-yaml)
-├── assets/
-│   └── logo.png    # Optional org logo
-└── CLAUDE.md
-```
+Single-file app. CSS in `<style>`, JS in `<script>`. No framework, no build tools. Vanilla JS with DOM manipulation (no re-rendering — elements are created once, updated via event listeners). Two CDN dependencies loaded at runtime:
 
-Single-file app. CSS is in `<style>`, JS is in `<script>`. No framework, no build tools. Vanilla JS with DOM manipulation (no re-rendering — elements are created once, updated via event listeners).
+- **pdf-lib** 1.17.1 — PDF creation, image embedding, PDF merging (`unpkg.com`)
+- **js-yaml** 4.1.0 — config.yml parsing (`unpkg.com`)
 
-## Dependencies (CDN, loaded at runtime)
+## Code sections (index.html)
 
-- **pdf-lib** (1.17.1) — PDF creation, image embedding, PDF merging. Loaded from unpkg.
-- **js-yaml** (4.1.0) — config.yml parsing. Loaded from unpkg.
-
-No npm, no node_modules, no bundler.
-
-## Key design decisions
-
-- **Custom currency dropdown** (`makeCDD`): native `<select>` can't show two-line options (code + name) that collapse to code-only. Built as a positioned div with click-outside-to-close.
-- **FX rate direction**: "units of line currency per 1 base currency" (e.g. 32.00000 THB per 1 USD). Conversion: `base_amount = line_amount / fx_rate`. Rate field locks to 1.00000 when line currency = base currency.
-- **Receipt storage**: `File.arrayBuffer()` stored in state. Memory-heavy for many large files but acceptable for typical reimbursement volumes (10-20 receipts).
-- **PDF page references**: two-pass approach. First pass renders form pages and records placeholder positions for "See page X" text. Second pass adds receipt pages, then backfills page numbers at recorded positions. Footers (Page X/Y) added last after total page count is known.
-- **Period auto-fill**: previous month by default; current month if today is the last day of the month.
-- **Staff name**: cached in `localStorage` under key `reimb-staff`.
+| Banner | Approx lines | Contents |
+|---|---|---|
+| `UTILITIES` | 103–131 | `uid()`, `el()`, `fmtAmt()`, `defaultPeriod()` |
+| `CONFIG` | 136–143 | `loadConfig()` — fetches and parses config.yml |
+| `STATE` | 145–155 | `state` object, `newItem()`, `newLine()` |
+| `CALCULATIONS` | 157–175 | `recalc()` |
+| `CURRENCY DROPDOWN` | 177–195 | `makeCDD()` |
+| `SELECT HELPER` | 197–208 | `makeSelect()` |
+| `FORM RENDERING` | 210–469 | `render()`, `renderItem()`, `renderLine()`, `buildReceiptArea()` |
+| `VALIDATION` | 471–500 | `validate()` |
+| `PDF ENGINE` | 502–786 | `generatePDF()` and helpers |
+| `GENERATE HANDLER` | 831–854 | `onGenerate()` |
+| `INIT` | 856–869 | `init()` |
 
 ## State model
 
-```
-state.staff         string
-state.periodFrom    string (YYYY-MM-DD)
-state.periodTo      string (YYYY-MM-DD)
-state.items[]
-  .id               string (uid)
-  .name             string
-  ._subtotal        number (calculated, base currency)
-  .lines[]
-    .id             string (uid)
-    .date           string (YYYY-MM-DD)
-    .description    string
-    .currency       string (ISO code)
-    .fxRate         string (5 decimal places)
-    .vendor         string
-    .hasReceipt     boolean
-    .receipts[]     { name, type, data: ArrayBuffer }
-    .noReceiptExplanation  string
-    .amount         string
-    .account        string
-    .program        string
-    .programOther   string
+```js
+state = {
+  staff: string,           // persisted to localStorage('reimb-staff')
+  periodFrom: string,      // YYYY-MM-DD
+  periodTo: string,        // YYYY-MM-DD
+  baseCurrency: string,    // ISO code; from CFG['currency-base']
+  fxRateMemory: {},        // { [code]: '00.00000' } session cache
+  items: Item[],
+  _grandTotal: number      // computed by recalc()
+}
+
+Item = {
+  id: string,
+  name: string,
+  lines: Line[],
+  _subtotal: number        // computed by recalc()
+}
+
+Line = {
+  id: string,
+  date: string,            // YYYY-MM-DD
+  description: string,
+  currency: string,        // ISO code
+  fxRate: string,          // '0.00000'; units of line currency per 1 base currency
+  vendor: string,
+  hasReceipt: boolean,
+  receipts: Receipt[],
+  noReceiptExplanation: string,
+  amount: string,          // in line currency
+  account: string,         // from CFG.accounts[]
+  program: string,         // from CFG.programs[]
+  programOther: string     // used when program === 'Other'
+}
+
+Receipt = {
+  name: string,
+  type: string,            // 'application/pdf' | 'image/png' | 'image/jpeg'
+  data: ArrayBuffer
+}
 ```
 
-No reactivity system. State is mutated directly by event listeners; `recalc()` is called explicitly after any change that affects totals.
+State is mutated directly. After any change that affects totals, call `recalc()` explicitly.
 
-## config.yml schema
+## Config keys — full reference
 
-| Key | Type | Notes |
-|-----|------|-------|
-| `organization` | string | Shown if logo disabled or missing |
-| `logo` | `yes`/`no` | Loads `assets/logo.png`, falls back to `.jpg` |
-| `logo-maxwidth` | number | cm — applied to both UI and PDF |
-| `page-size` | `A4`/`letter` | PDF output size |
-| `font-body`, `font-heading`, `font-monospace` | string | `Helvetica`/`Times`/`Courier` or `.ttf` path (TTF not yet implemented) |
-| `font-size` | number | Base pt size for PDF |
-| `accent-colour` | hex string | Applied as CSS variable and PDF heading/divider color |
-| `intro` | string | Rendered on PDF above form fields |
-| `footer` | string | Bottom of every PDF page |
-| `currency-base` | string | ISO code, must appear in currencies list |
-| `currencies[]` | `{code, name}` | Populates currency dropdowns |
-| `accounts[]` | string[] | Dropdown options |
-| `programs[]` | string[] | `"Other"` triggers a text field |
+All config is accessed as `CFG['key']` after `jsyaml.load()`. Keys with hyphens must use bracket notation.
 
-## PDF layout (pdf-lib coordinate system)
+| JS access | YAML key | Type | Default | Notes |
+|---|---|---|---|---|
+| `CFG.organization` | `organization` | string | — | Org name; PDF header and logo fallback |
+| `CFG.logo` | `logo` | `yes`/`no` | — | Checked as `=== true \|\| === 'yes'`; loads `assets/logo.png` then `assets/logo.jpg` |
+| `CFG['logo-maxwidth']` | `logo-maxwidth` | number (cm) | unconstrained | Converted to pt by `× 28.3465` for PDF; applied as `maxWidth` CSS in UI |
+| `CFG['page-size']` | `page-size` | `'A4'`/`'letter'` | `'A4'` | A4: 595.28×841.89 pt; letter: 612×792 pt |
+| `CFG['font-body']` | `font-body` | string | `'Helvetica'` | Parsed but currently unused (hardcoded to StandardFonts) |
+| `CFG['font-heading']` | `font-heading` | string | `'Helvetica'` | Parsed but currently unused |
+| `CFG['font-monospace']` | `font-monospace` | string | `'Courier'` | Parsed but currently unused |
+| `CFG['font-size']` | `font-size` | number (pt) | `10` | PDF base size; `szSm = sz-1`, `szLg = sz+4`, `lh = sz+4` |
+| `CFG['accent-colour']` | `accent-colour` | hex string | `'#1a3a5c'` | Set as CSS `--accent`; converted via `parseHex()` for PDF |
+| `CFG.intro` | `intro` | string | `''` | Rendered on PDF page 1 before header fields; omitted if empty |
+| `CFG.footer` | `footer` | string | `''` | Bottom of every PDF page |
+| `CFG['currency-base']` | `currency-base` | ISO code | — | Initial `state.baseCurrency`; must be in `currencies` list |
+| `CFG.currencies` | `currencies` | `{code, name}[]` | — | Options for per-line currency dropdown |
+| `CFG.accounts` | `accounts` | `string[]` | — | Options for account dropdown; selection required per line |
+| `CFG.programs` | `programs` | `string[]` | — | Options for program dropdown; literal `'Other'` triggers text input |
 
-- Origin: bottom-left. Y increases upward.
-- A4: 595.28 × 841.89 pt. Letter: 612 × 792 pt.
-- Margins: top 50, bottom 65, left 50, right 50.
-- Cursor (`y`) starts at `pageHeight - marginTop`, decrements downward.
-- `needSpace(h)` checks if `y - h < marginBottom`; if so, calls `addPage()` which draws the continuation header.
-- Fonts: StandardFonts only (Helvetica, HelveticaBold, Courier). Custom TTF embedding is stubbed in config but not yet wired.
+## Validation rules
 
-## Common tasks
+`validate()` returns `string[]`. Empty array means valid.
 
-**Add a new form field**: add to `newLine()` defaults → add UI in `renderLine()` → add validation in `validate()` → add PDF rendering in `generatePDF()`.
+- `state.staff` — required (non-empty after trim)
+- `state.periodFrom`, `state.periodTo` — both required
+- `state.items.length > 0` — at least one item
+- Per item: `name` required; `lines.length > 0`
+- Per line:
+  - `date` — required
+  - `description` — required
+  - `vendor` — required
+  - `amount` — required; must parse as positive float
+  - `fxRate` — required positive float if `currency !== baseCurrency`
+  - `account` — required
+  - `program` — required
+  - if `program === 'Other'`: `programOther` required
+  - if `hasReceipt === true`: `receipts.length > 0`
+  - if `hasReceipt === false`: `noReceiptExplanation` required
 
-**Change PDF layout**: all layout happens in `generatePDF()`. Column positions are defined as proportions of usable width (`W`). Adjust `c1`–`c4` and `r2v`/`r2r`/`r2a` variables.
+## PDF layout
 
-**Add a new config option**: add to `config.yml` → read from `CFG['key-name']` in JS.
+**Coordinate system:** origin bottom-left, Y increases upward.
 
-## Deployment
+**Margins:** top 50, bottom 65, left 50, right 50 (pt).
 
-Static files served from a web server. No build step. Drop the directory at the target path (e.g. `app.capthailand.org/reimbursement/`). Ensure the server serves `.yml` files with a valid MIME type (Caddy does this by default).
+**Usable width:** `W = pageW - 100`.
+
+**Column positions** (inside line blocks):
+```
+c1 = 0          // Date
+c2 = W * 0.22   // Vendor
+c3 = W * 0.68   // Currency, Receipt label
+c4 = W * 0.82   // FX rate, Amount label
+```
+Program: `W * 0.5`. Account: left edge.
+
+**Header columns:** Staff at left, Period at `W * 0.5`, Currency at `W * 0.8`.
+
+**Page break:** `needSpace(h)` checks `y - h < marginBottom`; if so, calls `addPage()` which draws a continuation header (staff name + period + divider).
+
+**Four-pass PDF build:**
+1. Form pages + receipt placeholder recording (`receiptRefs[]`)
+2. Receipt pages — PDF merging and image embedding (`receiptPageMap{}`)
+3. Backfill receipt references ("See page N for receipt")
+4. Footers on all pages (page X/Y, printed timestamp, staff name)
+
+**Downloaded filename:** `reimbursement_[staff]_[from]_[to].pdf` (spaces replaced with `_`).
+
+## FX rate convention
+
+Rate = units of line currency per 1 base currency.
+
+```
+base_amount = line_amount / fxRate
+```
+
+Example: 1000 THB at rate 34.25000 → 1000/34.25 = 29.20 USD.
+
+The `fxRate` field is locked to `'1.00000'` (readonly) when `line.currency === state.baseCurrency`. FX rates are cached in `state.fxRateMemory[code]` while the page is open.
+
+## Key design decisions
+
+**`makeCDD` — custom currency dropdown:** native `<select>` cannot show code+name two-line options that collapse to code-only. Built as a positioned div with click-outside-to-close via a document-level listener.
+
+**Receipt storage as ArrayBuffer:** files are stored raw in state, ready for pdf-lib without re-reading. Memory is proportional to total file size.
+
+**No re-rendering:** DOM is mutated in-place. Exception: `buildReceiptArea()` replaces the receipt area div when file list or toggle changes.
+
+**Period auto-fill:** `defaultPeriod()` returns the previous calendar month. Exception: if today is the last day of the month, returns the current month.
+
+**localStorage key:** `reimb-staff` — staff name only; no other state is persisted.
+
+## Common tasks (checklist)
+
+**Add a field to an expense line:**
+1. `newLine()` — add field with default
+2. `renderLine()` — create DOM, wire event listener, call `recalc()` if it affects amounts
+3. `validate()` — add error if required
+4. `generatePDF()` — render value at appropriate position
+
+**Add a config option:**
+1. Add to `config.yml`
+2. Read as `CFG['your-key']` in JS
+3. No other registration needed
+
+**Change PDF column layout:**
+Edit `c1`–`c4` constants inside the `item.lines.forEach` block in `generatePDF()`.
+
+**Implement custom TTF fonts:**
+1. `const bytes = await fetch('assets/Font.ttf').then(r => r.arrayBuffer())`
+2. `const font = await doc.embedFont(bytes)`
+3. Replace `StandardFonts.Helvetica` references with the embedded font
 
 ## Known limitations
 
-- Custom TTF fonts referenced in config are not yet embedded in PDFs (falls back to standard fonts)
-- Password-protected PDF receipts will fail to merge
-- No offline support (CDN dependencies)
-- Large receipt volumes may stress browser memory
-- PDF form layout has fixed column proportions — very long field values get truncated with `…`
+| Limitation | Detail |
+|---|---|
+| Custom fonts not embedded | `font-body`/`font-heading`/`font-monospace` config keys parsed but ignored; hardcoded to Helvetica/Courier |
+| Password-protected PDFs fail | pdf-lib cannot decrypt; error page inserted instead |
+| No offline support | CDN dependencies require network on first load |
+| Large receipts stress memory | All ArrayBuffers held in JS heap |
+| Long text truncated with `…` | Most PDF fields truncate to fit column width; only `intro` and `noReceiptExplanation` wrap |
+
+## Deployment
+
+Static files served from any HTTP server. No build step. Place `app/` at the target path. The server must serve `.yml` files (Caddy does this by default; for Nginx add `text/yaml yml` to `types {}`).
+
+Local development: `python3 -m http.server 8080` from the `app/` directory. Do not open `index.html` via `file://` — `fetch('config.yml')` is blocked on that origin.
+
+## Human-readable docs
+
+- `docs/user-guide.md` — filling in the form (staff audience)
+- `docs/admin-guide.md` — deployment and all config.yml keys (admin audience)
+- `docs/developer-guide.md` — architecture, state, PDF engine, modification patterns (developer audience)
diff --git a/docs/admin-guide.md b/docs/admin-guide.md
new file mode 100644
index 0000000..175f356
--- /dev/null
+++ b/docs/admin-guide.md
@@ -0,0 +1,200 @@
+# Administrator Guide — Reimbursement Form
+
+This guide is for the person responsible for deploying and configuring the reimbursement form for their organisation.
+
+---
+
+## Architecture
+
+The application is a static web app — a single HTML file that loads a YAML configuration file at runtime. There is no server-side logic, no database, and no build step.
+
+```
+app/
+├── index.html        # Complete application
+├── config.yml        # All organisation-specific settings
+└── assets/
+    └── logo.png      # Optional organisation logo (PNG or JPG)
+```
+
+The browser downloads the two CDN libraries at startup:
+- `pdf-lib` 1.17.1 — PDF generation
+- `js-yaml` 4.1.0 — YAML config parsing
+
+Both are loaded from `unpkg.com`. Users need an internet connection to open the form for the first time; subsequent use within the same browser session does not re-fetch them.
+
+---
+
+## Deployment
+
+1. Copy the `app/` directory to any static web server (Nginx, Caddy, Apache, S3 + CloudFront, GitHub Pages, etc.).
+2. Ensure the server serves `.yml` files with a valid MIME type. Caddy does this automatically. For Nginx, add:
+   ```nginx
+   types { text/yaml yml; }
+   ```
+3. Access the form at the URL where `index.html` is served. No path configuration is needed inside the file.
+
+The form works with any URL path. If you deploy at `https://intranet.example.org/finance/reimbursement/`, no changes to the source are required.
+
+---
+
+## Configuration reference — `config.yml`
+
+All customisation is done in `config.yml`. The file is loaded fresh every time the page is opened, so changes take effect immediately without redeploying.
+
+### Organisation identity
+
+```yaml
+organization: "Center for Asylum Protection"
+logo: yes
+logo-maxwidth: 4
+```
+
+| Key | Type | Required | Description |
+|---|---|---|---|
+| `organization` | string | Yes | Displayed in the form header and on the PDF. Used as a fallback if the logo is disabled or the image file is missing. |
+| `logo` | `yes` / `no` | Yes | Whether to show a logo image. If `yes`, the app attempts to load `assets/logo.png`; if that fails, it tries `assets/logo.jpg`; if that also fails, it falls back to displaying the `organization` text. |
+| `logo-maxwidth` | number (cm) | No | Maximum width of the logo in centimetres, applied in both the browser UI and the PDF. Default behaviour is unconstrained. Typical value: `4`. |
+
+**Logo requirements:**
+- Format: PNG (preferred) or JPG
+- File must be placed at `assets/logo.png` (or `assets/logo.jpg`)
+- The image is scaled to fit within `logo-maxwidth` cm width and 56 px height (UI) or 50 pt height (PDF), whichever is the binding constraint
+
+### PDF page setup
+
+```yaml
+page-size: A4
+```
+
+| Key | Type | Required | Description |
+|---|---|---|---|
+| `page-size` | `A4` / `letter` | Yes | Paper size for PDF output. `A4` = 595.28 × 841.89 pt. `letter` = 612 × 792 pt. |
+
+### Typography
+
+```yaml
+font-body: Helvetica
+font-heading: Helvetica
+font-monospace: Courier
+font-size: 10
+```
+
+| Key | Type | Required | Description |
+|---|---|---|---|
+| `font-body` | string | Yes | Font used for body text in the PDF. Must be one of the standard PDF fonts: `Helvetica`, `Times`, or `Courier`. Custom TTF fonts are not yet supported. |
+| `font-heading` | string | Yes | Font used for headings and labels in the PDF. Same options as `font-body`. |
+| `font-monospace` | string | Yes | Font used for monospaced content. Recommended: `Courier`. |
+| `font-size` | number (pt) | Yes | Base font size in points for PDF body text. Headings and labels are scaled relative to this. Typical range: 9–12. |
+
+> **Note on fonts:** The current implementation always uses `Helvetica`/`HelveticaBold`/`Courier` regardless of what `font-body`, `font-heading`, and `font-monospace` are set to. Custom font selection is planned but not yet implemented.
+
+### Branding
+
+```yaml
+accent-colour: "#1a3a5c"
+```
+
+| Key | Type | Required | Description |
+|---|---|---|---|
+| `accent-colour` | hex colour string | Yes | Primary brand colour. Applied as the `--accent` CSS variable in the browser UI (buttons, borders, headings) and as the heading/divider colour in the PDF. Must be a six-digit hex string including the `#` prefix. |
+
+### Form text
+
+```yaml
+intro: ""
+footer: "Confidential"
+```
+
+| Key | Type | Required | Description |
+|---|---|---|---|
+| `intro` | string | No | Optional introductory text shown on the first page of the PDF, above the staff/period/currency fields. Leave empty (`""`) to omit. Long text wraps automatically. |
+| `footer` | string | No | Text printed at the bottom of every PDF page. Typical values: `"Confidential"`, `"Internal use only"`, or your organisation's name. |
+
+### Currency settings
+
+```yaml
+currency-base: USD
+
+currencies:
+  - code: USD
+    name: United States dollar
+  - code: THB
+    name: Thai baht
+  - code: EUR
+    name: Euro
+  - code: NOK
+    name: Norwegian krone
+```
+
+| Key | Type | Required | Description |
+|---|---|---|---|
+| `currency-base` | ISO 4217 code | Yes | The default base (reimbursement) currency. Must be present in the `currencies` list. All amounts on the PDF summary are converted to this currency. |
+| `currencies` | list of `{code, name}` | Yes | All currencies available in the per-line currency dropdown. `code` must be an ISO 4217 three-letter code. `name` is shown in the dropdown for user reference only. |
+
+**FX rate convention:** rates are expressed as units of the line currency per 1 unit of the base currency (e.g. 34.25 THB per 1 USD). Conversion is `base_amount = line_amount / fx_rate`.
+
+**Adding a currency:** append a new `{code, name}` entry to `currencies`. No other changes are needed.
+
+**Removing a currency:** remove its entry. Any existing PDF claims already generated are unaffected.
+
+### Accounts
+
+```yaml
+accounts:
+  - "1000 - General Operations"
+  - "2000 - Travel & Transport"
+  - "3000 - Office Supplies"
+  - "4000 - Professional Services"
+```
+
+| Key | Type | Required | Description |
+|---|---|---|---|
+| `accounts` | list of strings | Yes | Dropdown options for the "Account" field on each expense line. Each string is shown verbatim. Typically formatted as `"CODE - Description"`. At least one entry is required. |
+
+Selecting an account is mandatory for each line. The account code selected appears verbatim on the generated PDF.
+
+### Programs
+
+```yaml
+programs:
+  - "General Operations"
+  - "Legal Aid Program"
+  - "Protection Program"
+  - Other
+```
+
+| Key | Type | Required | Description |
+|---|---|---|---|
+| `programs` | list of strings | Yes | Dropdown options for the "Program" field on each expense line. At least one entry is required. |
+
+**Special value `Other`:** if the literal string `Other` is included in the list, selecting it in the form reveals an additional free-text field where the user must type the program name. The PDF will show `Other: [typed value]`. All other values are shown exactly as written.
+
+---
+
+## Changing configuration
+
+1. Edit `app/config.yml` in any text editor.
+2. Save the file and refresh the browser. Changes are live immediately.
+3. Existing downloaded PDFs are not affected.
+
+---
+
+## Adding a logo
+
+1. Prepare a PNG or JPG image of your logo. Transparent background (PNG) works best.
+2. Place the file at `app/assets/logo.png` (create the `assets/` folder if it does not exist).
+3. Set `logo: yes` in `config.yml`.
+4. Set `logo-maxwidth` to the desired maximum width in centimetres.
+
+---
+
+## Troubleshooting
+
+| Problem | Cause | Fix |
+|---|---|---|
+| Form shows "Failed to load: Cannot load config.yml" | Server not serving the YAML file, or file missing | Check the file path and server MIME type configuration |
+| Logo does not appear | File missing or wrong path | Place file at `assets/logo.png` relative to `index.html` |
+| Accent colour not applied | Malformed hex value | Use the format `"#rrggbb"` (six digits, with quotes, with `#`) |
+| Currency not in dropdown | Missing from `currencies` list | Add the `{code, name}` entry to `currencies` |
+| `currency-base` not selectable | Code not in `currencies` list | Add it to `currencies` before using it as `currency-base` |
+| PDF fonts look wrong | Custom font specified | Only `Helvetica`, `Times`, `Courier` are currently supported |
diff --git a/docs/developer-guide.md b/docs/developer-guide.md
new file mode 100644
index 0000000..173131e
--- /dev/null
+++ b/docs/developer-guide.md
@@ -0,0 +1,276 @@
+# Developer Guide — Reimbursement Form
+
+This guide is for developers who want to fork, extend, or modify the reimbursement form.
+
+---
+
+## Repository layout
+
+```
+.
+├── CLAUDE.md               # Agent-oriented reference (architecture, state, tasks)
+├── README.md
+├── LICENSE
+├── docs/
+│   ├── user-guide.md
+│   ├── admin-guide.md
+│   └── developer-guide.md  # this file
+└── app/
+    ├── index.html          # Entire application (~870 lines)
+    ├── config.yml          # Runtime configuration
+    └── assets/
+        └── logo.png        # Optional organisation logo
+```
+
+---
+
+## Technology
+
+| Concern | Technology |
+|---|---|
+| UI | Vanilla JS — DOM manipulation, no framework, no virtual DOM |
+| PDF generation | [pdf-lib](https://pdf-lib.js.org/) 1.17.1 via CDN |
+| Config parsing | [js-yaml](https://github.com/nodeca/js-yaml) 4.1.0 via CDN |
+| Build | None — single HTML file served as-is |
+
+There is no `package.json`, no bundler, and no transpilation step. The file runs directly in any modern browser.
+
+---
+
+## Code structure inside `index.html`
+
+The JS is one immediately-invoked async function. Sections are separated by `// ===` banners:
+
+| Section | Line range | Purpose |
+|---|---|---|
+| UTILITIES | ~103–131 | `uid()`, `el()`, `fmtAmt()`, `defaultPeriod()` |
+| CONFIG | ~136–143 | `loadConfig()` — fetches and parses `config.yml` |
+| STATE | ~145–155 | `state` object, `newItem()`, `newLine()` |
+| CALCULATIONS | ~157–175 | `recalc()` — recomputes subtotals and grand total |
+| CURRENCY DROPDOWN | ~177–195 | `makeCDD()` — custom two-line currency picker |
+| SELECT HELPER | ~197–208 | `makeSelect()` — standard `<select>` builder |
+| FORM RENDERING | ~210–469 | `render()`, `renderItem()`, `renderLine()`, `buildReceiptArea()` |
+| VALIDATION | ~471–500 | `validate()` — returns array of error strings |
+| PDF ENGINE | ~502–786 | `generatePDF()` and helpers |
+| GENERATE HANDLER | ~831–854 | `onGenerate()` — wires validation to PDF generation |
+| INIT | ~856–869 | `init()` — entry point |
+
+---
+
+## State model
+
+```js
+state = {
+  staff: string,            // Full name, persisted in localStorage('reimb-staff')
+  periodFrom: string,       // YYYY-MM-DD
+  periodTo: string,         // YYYY-MM-DD
+  baseCurrency: string,     // ISO code from CFG['currency-base']
+  fxRateMemory: {},         // { [currencyCode]: '00.00000' } — session cache
+  items: Item[],
+  _grandTotal: number       // computed by recalc()
+}
+
+Item = {
+  id: string,               // uid()
+  name: string,
+  lines: Line[],
+  _subtotal: number         // computed by recalc()
+}
+
+Line = {
+  id: string,               // uid()
+  date: string,             // YYYY-MM-DD
+  description: string,
+  currency: string,         // ISO code
+  fxRate: string,           // '0.00000' — units of line currency per 1 base currency
+  vendor: string,
+  hasReceipt: boolean,
+  receipts: Receipt[],
+  noReceiptExplanation: string,
+  amount: string,           // in line currency
+  account: string,          // one of CFG.accounts[]
+  program: string,          // one of CFG.programs[]
+  programOther: string      // used when program === 'Other'
+}
+
+Receipt = {
+  name: string,             // original filename
+  type: string,             // MIME type: 'application/pdf', 'image/png', 'image/jpeg'
+  data: ArrayBuffer         // raw file bytes
+}
+```
+
+State is mutated directly — there is no reactivity system. After any change that affects totals, call `recalc()` explicitly.
+
+---
+
+## Config keys read by the JS
+
+| JS expression | Config key | Type | Notes |
+|---|---|---|---|
+| `CFG['accent-colour']` | `accent-colour` | string | Hex colour applied via CSS variable and used in PDF |
+| `CFG['page-size']` | `page-size` | string | `'A4'` or `'letter'` |
+| `CFG['font-size']` | `font-size` | number | Base pt size; `sz` in PDF engine |
+| `CFG['font-body']` | `font-body` | string | Parsed but currently unused (hardcoded to Helvetica) |
+| `CFG['font-heading']` | `font-heading` | string | Parsed but currently unused |
+| `CFG['font-monospace']` | `font-monospace` | string | Parsed but currently unused |
+| `CFG.logo` | `logo` | `true` / `'yes'` / `false` / `'no'` | Checked with `=== true \|\| === 'yes'` |
+| `CFG['logo-maxwidth']` | `logo-maxwidth` | number | cm; converted to pt by `× 28.3465` |
+| `CFG.organization` | `organization` | string | Fallback text and PDF org header |
+| `CFG['currency-base']` | `currency-base` | string | Initial value for `state.baseCurrency` |
+| `CFG.currencies` | `currencies` | `{code, name}[]` | Currency dropdown options |
+| `CFG.accounts` | `accounts` | `string[]` | Account dropdown options |
+| `CFG.programs` | `programs` | `string[]` | Program dropdown options |
+| `CFG.intro` | `intro` | string | Rendered on first PDF page; empty string omits it |
+| `CFG.footer` | `footer` | string | Printed at bottom of every PDF page |
+
+---
+
+## PDF engine
+
+### Coordinate system
+
+pdf-lib uses a bottom-left origin. Y increases upward.
+
+| Page size | Width (pt) | Height (pt) |
+|---|---|---|
+| A4 | 595.28 | 841.89 |
+| letter | 612 | 792 |
+
+Margins: top 50 pt, bottom 65 pt, left 50 pt, right 50 pt.
+
+The cursor variable `y` starts at `pageH - marginTop` and decrements as content is drawn. `needSpace(h)` checks `y - h < marginBottom` and calls `addPage()` if a page break is needed.
+
+### Column positions
+
+Within each expense line the usable width `W = pageW - 100` is divided using fixed proportions:
+
+```js
+const c1 = 0;         // Date column — left edge
+const c2 = W * 0.22;  // Vendor column
+const c3 = W * 0.68;  // Currency / Receipt columns
+const c4 = W * 0.82;  // FX rate / Amount columns
+```
+
+Program label is at `W * 0.5`. Account starts at `c1`.
+
+### Two-pass receipt page reference
+
+The PDF is built in four passes:
+
+1. **Form pages:** draw all item/line content. For each receipt, record `{ pageIdx, x, y, key }` in `receiptRefs[]` — placeholder positions.
+2. **Receipt pages:** append embedded PDFs (page by page) and images (scaled to fit). Build `receiptPageMap[key] = pageNumber`.
+3. **Reference backfill:** go back to each recorded position and draw `"See page N for receipt"`.
+4. **Footers:** iterate all pages and draw the footer line with page X/Y and printed timestamp.
+
+### Fonts
+
+Three fonts are embedded from the standard PDF font set:
+
+```js
+const fontBody = await doc.embedFont(StandardFonts.Helvetica);
+const fontBold = await doc.embedFont(StandardFonts.HelveticaBold);
+const fontMono = await doc.embedFont(StandardFonts.Courier);
+```
+
+Custom TTF embedding is not yet wired despite the config keys being present.
+
+### Font sizes
+
+```js
+const sz   = CFG['font-size'] || 10;   // body
+const szSm = sz - 1;                   // labels
+const szLg = sz + 4;                   // title, org name
+const lh   = sz + 4;                   // line height
+```
+
+---
+
+## Common modification tasks
+
+### Add a new field to an expense line
+
+Follow this checklist in order:
+
+1. **`newLine()`** — add the field with its default value.
+2. **`renderLine()`** — create and wire the DOM element; update state in an event listener; call `recalc()` if the field affects amounts.
+3. **`validate()`** — add an error string if the field is required.
+4. **`generatePDF()`** — render the field value at the appropriate position.
+
+### Add a new config option
+
+1. Add the key and value to `config.yml`.
+2. Read it in JS as `CFG['your-key']` (or `CFG.yourKey` if it has no hyphens).
+3. No other registration is needed — `jsyaml.load()` makes all keys available.
+
+### Change PDF column proportions
+
+Edit the `c1`–`c4` constants inside the `item.lines.forEach` block in `generatePDF()`. They are local to that scope. The header layout (Staff/Period/Currency) uses separate constants `col2 = W * 0.5` and `col3 = W * 0.8`.
+
+### Implement custom TTF fonts
+
+The config keys `font-body`, `font-heading`, `font-monospace` are parsed but not used. To implement:
+
+1. Fetch the TTF file: `const fontBytes = await fetch('assets/MyFont.ttf').then(r => r.arrayBuffer())`.
+2. Embed it: `const fontBody = await doc.embedFont(fontBytes)`.
+3. Replace the `StandardFonts.Helvetica` call with the embedded font.
+4. Do the same for bold and monospace variants.
+
+Note: pdf-lib requires the font to be a valid OpenType/TrueType file. Subset embedding is done automatically.
+
+### Add a new top-level form field (not per-line)
+
+1. Add the field to `state` in the module-level `state` declaration.
+2. Render it in `render()`, between the header and the divider.
+3. Add validation in `validate()`.
+4. Render it in `generatePDF()` in the header section (around the Staff/Period/Currency row).
+
+---
+
+## Key design decisions
+
+**Custom currency dropdown (`makeCDD`):** The native `<select>` element cannot display two-line options (code on line 1, name on line 2) that collapse to code-only in the closed state. The custom dropdown is a positioned `div` with click-outside-to-close via a document-level listener.
+
+**FX rate direction:** The rate is "units of line currency per 1 base currency". This means `base_amount = line_amount / fx_rate`. Example: if base is USD and line is THB at rate 34.25, then 1000 THB = 1000/34.25 = 29.20 USD.
+
+**Receipt storage as ArrayBuffer:** Files are stored as `ArrayBuffer` in state (not as object URLs or base64). This keeps them ready for pdf-lib without re-reading. Memory use is proportional to total receipt file size.
+
+**No re-rendering:** DOM elements are created once by `render()`/`renderItem()`/`renderLine()` and mutated in-place by event listeners. The only exception is the receipt area (`buildReceiptArea`), which is replaced when the file list or receipt toggle changes.
+
+**`localStorage` for staff name:** Keyed as `reimb-staff`. This persists the most frequently retyped field across sessions without requiring a server.
+
+**Period default logic:** `defaultPeriod()` returns the previous calendar month in most cases. Exception: if today is the last day of the month, it returns the current month (on the assumption that the user is filing for the month just ending).
+
+---
+
+## Known limitations and planned work
+
+| Limitation | Notes |
+|---|---|
+| Custom TTF fonts not embedded | Config keys present but code uses StandardFonts only |
+| Password-protected PDF receipts fail | pdf-lib cannot decrypt; an error page is inserted instead |
+| No offline support | CDN-loaded libraries require internet on first load |
+| Large receipts stress browser memory | ArrayBuffer approach stores all bytes in RAM |
+| PDF text truncated with `…` | Long field values are cut to fit the fixed column width |
+| No multi-line PDF text in most fields | Only `intro` and `noReceiptExplanation` wrap; other fields truncate |
+
+---
+
+## Testing
+
+There is no automated test suite. To test manually:
+
+1. Open `app/index.html` via a local HTTP server (e.g. `python3 -m http.server 8080` from the `app/` directory, then open `http://localhost:8080`).
+2. Opening `index.html` as a `file://` URL will fail — `fetch('config.yml')` is blocked by browser security on `file://` origins.
+3. Exercise the form: add items and lines, attach receipts (PDF, PNG, JPG), toggle the receipt Yes/No, use non-base currencies, and generate a PDF.
+4. Verify the generated PDF: check totals, receipt embedding, page references, footers, and column layout.
+
+---
+
+## Deployment checklist
+
+- [ ] `config.yml` is updated for the target organisation
+- [ ] `assets/logo.png` placed if `logo: yes`
+- [ ] Web server serves `.yml` with a valid MIME type (`text/yaml`)
+- [ ] URL is accessible to intended users
+- [ ] Browser test: open form, fill in one claim, generate PDF, verify output
diff --git a/docs/user-guide.md b/docs/user-guide.md
new file mode 100644
index 0000000..10f11f5
--- /dev/null
+++ b/docs/user-guide.md
@@ -0,0 +1,117 @@
+# User Guide — Reimbursement Form
+
+This guide is for staff submitting expense reimbursement claims.
+
+---
+
+## Overview
+
+The reimbursement form runs in your browser. You fill in your expenses, attach receipts, and click one button to download a PDF. No account, login, or internet upload is needed — the PDF is generated entirely on your device.
+
+---
+
+## Step-by-step
+
+### 1. Open the form
+
+Navigate to the URL your administrator has given you (e.g. `https://app.example.org/reimbursement/`). The form loads in a few seconds. If you see an error, check your internet connection — the form needs to reach CDN servers to load its libraries.
+
+### 2. Fill in the header fields
+
+At the top of the form you will see four fields:
+
+| Field | What to enter |
+|---|---|
+| **Staff** | Your full name. This is saved in your browser and pre-filled next time. |
+| **Period from** | Start date of the expense period (defaults to the first day of last month). |
+| **to** | End date of the expense period (defaults to the last day of last month). |
+| **Base currency** | The currency your reimbursement will be paid in. Select from the dropdown. |
+
+> **Period default logic:** On the last day of the current month, the period defaults to the current month instead of the previous one.
+
+### 3. Add items
+
+An "item" groups related expenses — for example, a trip, a project, or a category. Click **+ Add item / project / travel** to create one. Give the item a descriptive name (e.g. "Field visit to Chiang Rai, March 2026").
+
+You can add as many items as you need and remove any item with the **✕ Remove** button.
+
+### 4. Add expense lines
+
+Each item contains one or more expense lines. A line represents a single purchase or payment. Click **+ Add line** inside an item to add a line.
+
+Each line has the following fields:
+
+#### Date
+The date the expense was incurred. Use the date picker or type `YYYY-MM-DD`.
+
+#### Vendor
+The name of the shop, service provider, or payee. Example: "Bangkok Airways", "Office Depot".
+
+#### Currency
+The currency in which you paid. Select from the dropdown. Each option shows the currency code (e.g. `THB`) and the full name (e.g. "Thai baht").
+
+#### FX rate
+How many units of the line currency equal one unit of the base currency.
+
+- Example: if base currency is USD and you paid in THB, enter the number of Thai baht per 1 US dollar (e.g. `34.25000`).
+- If the line currency is the same as the base currency, this field is locked at `1.00000` and you cannot change it.
+- The form remembers FX rates you have entered for each currency during the session.
+
+> **FX rate direction:** `line_amount ÷ fx_rate = base_amount`. Always use the rate as "how many [line currency] per 1 [base currency]".
+
+#### Description
+A brief description of what was purchased or paid for. Example: "Taxi from airport to hotel".
+
+#### Receipt
+Select **Yes** if you have a receipt, **No** if you do not.
+
+- **Yes:** Upload one or more receipt files (PDF, JPG, or PNG). Each file will be embedded at the end of the PDF output. You can remove an uploaded file and re-upload.
+- **No:** A text box appears. You must explain why no receipt is available (e.g. "Toll fees — no receipt issued"). This explanation will appear on the PDF.
+
+#### Amount
+The amount paid, in the line currency (not converted). Enter a positive number. Example: `1850.00`.
+
+#### Account
+Select the accounting code that this expense belongs to. The options are configured by your administrator.
+
+#### Program
+Select the program or project this expense relates to. If you select **Other**, a text field appears where you must type the program name.
+
+### 5. Review totals
+
+Each item shows a **Subtotal** in the base currency. The bottom of the form shows the **Total reimbursement claim**. These update as you type.
+
+### 6. Generate the PDF
+
+Click **Generate Reimbursement Form**. The form validates all fields first. If anything is missing or incorrect, a red box at the top lists the problems — fix them and click again.
+
+When validation passes, the PDF is generated in your browser and downloaded automatically. The filename is:
+
+```
+reimbursement_[Your_Name]_[from-date]_[to-date].pdf
+```
+
+### 7. Submit the PDF
+
+Send the downloaded PDF to whoever handles reimbursements at your organisation. No further action is needed in the browser.
+
+---
+
+## Tips
+
+- **Your name is remembered.** It is stored in your browser's local storage so you do not need to retype it next visit.
+- **FX rates are remembered within a session.** If you enter a rate for THB and then switch to USD and back, the THB rate is restored.
+- **Large receipt files slow PDF generation.** If the browser appears frozen after clicking "Generate", wait — it is processing the files.
+- **Password-protected PDFs cannot be merged.** If a receipt PDF is password-protected, the output will contain an error page in its place. Use a non-protected version.
+
+---
+
+## Troubleshooting
+
+| Symptom | Likely cause | Fix |
+|---|---|---|
+| Form shows "Loading configuration…" indefinitely | Network issue or server misconfiguration | Check internet connection; contact your administrator |
+| "Failed to load: Cannot load config.yml" | Server is not serving the config file | Contact your administrator |
+| PDF download starts but contains error text | A receipt PDF is password-protected or corrupt | Replace the file with an unprotected version |
+| Amounts show as "–" in the PDF | Amount field left blank or non-numeric | Enter a valid number in the amount field |
+| FX rate field is greyed out | Line currency equals the base currency | No action needed; rate is 1.00000 by definition |