Add QR scanner to the send page (lazy-loaded decoder)

Scan a Bitcoin address, a BIP21 URI (fills address + amount), or a
signed-transaction QR (opens a broadcast confirmation, then relays it)
from a viewfinder button on the Send form.

Decoder strategy keeps the main bundle small:
- Prefer the native BarcodeDetector (Chrome desktop/Android) — no lib.
- Otherwise lazy-load jsQR from a separate dist/jsqr.js, only when the
  scanner is opened. jsQR stays out of index.html (~424 KB vs ~551 KB)
  and off the initial load.
- The service worker precaches jsqr.js so offline scanning works for
  installed PWAs on browsers without BarcodeDetector.

The scanner needs a camera + secure context, so it never shows on a
standalone file:// page; offline air-gapped signing is unaffected.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

1 files changed, 7 insertions, 2 deletions

diff --git a/dev.js b/dev.js
index 099069c..dde8813 100644
--- a/dev.js
+++ b/dev.js
@@ -1,14 +1,19 @@
 // Dev server: rebuilds the inlined bundle on every request so you just refresh
 // the browser to see changes. Same output as `bun run build`, unminified.
 
-import { buildHtml } from './build.js';
+import { buildHtml, buildJsQr } from './build.js';
 
 const port = Number(process.env.PORT || 5173);
 
 Bun.serve({
   port,
-  async fetch() {
+  async fetch(req) {
     try {
+      if (new URL(req.url).pathname === '/jsqr.js') {
+        return new Response(await buildJsQr({ minify: false }), {
+          headers: { 'content-type': 'text/javascript; charset=utf-8' },
+        });
+      }
       const html = await buildHtml({ minify: false });
       return new Response(html, {
         headers: { 'content-type': 'text/html; charset=utf-8' },