Fix receiver: stale closure + add GIF file test mode

- Fix stale closure bug in camera capture loop (scanning state
  captured as false, preventing any frame capture)
- Add GIF File input mode to receiver — upload a QR-over-GIF
  directly to test the decode pipeline without camera
- Update decode worker to accept imageData or frameData
- Receiver now shows two input modes: Camera and GIF File

src/workers/decode.worker.ts

@@ -53,8 +53,10 @@ self.onmessage = (e: MessageEvent) => {
   }
 
   if (msg.type === 'frame') {
+    const imageData: ImageData = msg.imageData ?? msg.frameData;
+    if (!imageData) return;
     try {
-      handleFrame(msg.imageData as ImageData);
+      handleFrame(imageData);
     } catch (err: any) {
       // Don't crash worker on decode errors
       self.postMessage({ type: 'error', message: err.message ?? String(err) });