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>

src/lib/barcodeDetectorSupport.ts

@@ -0,0 +1,35 @@
+import { BarcodeDetectorPolyfill } from "@undecaf/barcode-detector-polyfill";
+
+type WindowWithBarcodeDetector = Window & {
+  BarcodeDetector?: typeof BarcodeDetectorPolyfill;
+};
+
+let detectorReady: Promise<void> | null = null;
+
+export function isAppleMobileDevice() {
+  if (typeof navigator === "undefined") return false;
+  const ua = navigator.userAgent;
+  return /iPad|iPhone|iPod/i.test(ua) || (navigator.platform === "MacIntel" && navigator.maxTouchPoints > 1);
+}
+
+export function ensureBarcodeDetector() {
+  if (detectorReady) return detectorReady;
+
+  detectorReady = (async () => {
+    const globalWindow = window as WindowWithBarcodeDetector;
+    const shouldForcePolyfill = isAppleMobileDevice();
+
+    if (shouldForcePolyfill) {
+      globalWindow.BarcodeDetector = BarcodeDetectorPolyfill;
+      return;
+    }
+
+    try {
+      await globalWindow.BarcodeDetector?.getSupportedFormats();
+    } catch {
+      globalWindow.BarcodeDetector = BarcodeDetectorPolyfill;
+    }
+  })();
+
+  return detectorReady;
+}