Move all static assets into app/assets/

Favicons, icons, and site.webmanifest moved from app/ root to app/assets/.
Updated all href references in index.html accordingly.

https://claude.ai/code/session_01MkM7p8Us3L8YAfLKGA13NS

app/index.html

@@ -5,12 +5,12 @@
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   <meta name="format-detection" content="telephone=no">
   <title>Invoice</title>
-  <link rel="icon" href="favicon.svg" type="image/svg+xml">
-  <link rel="icon" href="favicon-48.png" sizes="48x48" type="image/png">
-  <link rel="icon" href="favicon-32.png" sizes="32x32" type="image/png">
-  <link rel="icon" href="favicon-16.png" sizes="16x16" type="image/png">
-  <link rel="apple-touch-icon" href="apple-touch-icon.png">
-  <link rel="manifest" href="site.webmanifest">
+  <link rel="icon" href="assets/favicon.svg" type="image/svg+xml">
+  <link rel="icon" href="assets/favicon-48.png" sizes="48x48" type="image/png">
+  <link rel="icon" href="assets/favicon-32.png" sizes="32x32" type="image/png">
+  <link rel="icon" href="assets/favicon-16.png" sizes="16x16" type="image/png">
+  <link rel="apple-touch-icon" href="assets/apple-touch-icon.png">
+  <link rel="manifest" href="assets/site.webmanifest">
   <meta name="theme-color" content="#2f6fed">
   <script>
     /* Restore persisted theme before first paint */