Replace browser print with jsPDF Helvetica PDF generation

The browser print-to-PDF approach requires the user to manually toggle
"Background graphics" on and "Headers and footers" off — impossible to
control programmatically. Replace with direct jsPDF generation:

- Loads jsPDF 2.5.1 UMD from CDN
- buildPDF() draws the invoice programmatically using Helvetica (PDF
  built-in font, no embedding needed)
- Layout: sender/title header, navy rule, charge-to block with navy
  accent bar, line-items table (navy header, alternating rows, multi-
  line description wrapping, foreign-currency sub-row), totals with
  navy To-Pay bar — all in mm on A4
- PDF downloads directly as invoice-{no}.pdf with no dialog
- HTML preview overlay retained for review before download
- Replace "print-invoice" config key with "download-pdf"

https://claude.ai/code/session_015iyCBgoTXNNqaErR287U1u

app/config.yml

@@ -389,11 +389,11 @@ translations:
     de: "Nein"
     fr: "Non"
     "no": "Nei"
-  print-invoice:
-    en: "Print / Save as PDF"
-    de: "Drucken / Als PDF speichern"
-    fr: "Imprimer / Enregistrer en PDF"
-    "no": "Skriv ut / Lagre som PDF"
+  download-pdf:
+    en: "Download PDF"
+    de: "PDF herunterladen"
+    fr: "Télécharger PDF"
+    "no": "Last ned PDF"
   close:
     en: Close
     de: Schließen