Fix barcode scanner on iOS Safari

iOS WebKit does not provide a reliable native Barcode Detection API, and
ZXing often failed due to strict camera constraints and video startup timing.

- Install @undecaf/barcode-detector-polyfill (ZBar WASM) on Apple devices
- Fall back through progressively looser getUserMedia constraints
- Wait for video metadata/playback before decoding frames
- Throttle native scans on iOS and tune ZXing retry intervals
- Defer scanner startup until the modal video element is mounted

Co-authored-by: Ned Halksworth <hello@nedhalksworth.com>

package.json

@@ -12,6 +12,7 @@
     "setup:appwrite": "node scripts/setup-appwrite.mjs"
   },
   "dependencies": {
+    "@undecaf/barcode-detector-polyfill": "^0.9.23",
     "@vitejs/plugin-react": "^4.3.4",
     "@zxing/browser": "^0.2.0",
     "appwrite": "^25.0.0",