feat(crypto): QR-code device verification (alongside emoji SAS)

B2 of the Matrix protocol-gaps roadmap, gate-green (688 tests):
- Enable QR verification methods (show/scan/reciprocate) in initMatrix.
- Extend DeviceVerification: the Ready step offers your own QR (byte-mode encode
  via qrcode), a camera 'Scan their QR code' flow, and an emoji fallback; the
  Started step routes reciprocate → a confirm step (useVerifierShowReciprocateQr)
  or SAS as before.
- New QrScanner component: getUserMedia + jsQR, handing the raw binaryData bytes
  to request.scanQRCode (BarcodeDetector is string-only, so can't be used).
- Adds qrcode + jsqr (small, pure-JS, client-only); build-verified under rolldown.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/components/QrScanner.tsx

@@ -0,0 +1,101 @@
+import React, { useEffect, useRef, useState } from 'react';
+import { Box, Button, color, config, Text } from 'folds';
+import jsQR from 'jsqr';
+
+type QrScannerProps = {
+  onScan: (bytes: Uint8ClampedArray) => void;
+  onCancel: () => void;
+};
+
+// Camera QR scanner. Decodes frames with jsQR and hands back the raw byte
+// segment (`result.binaryData`) — Matrix QR verification needs the raw bytes,
+// not a decoded string, so the string-only `BarcodeDetector` can't be used.
+export function QrScanner({ onScan, onCancel }: QrScannerProps) {
+  const videoRef = useRef<HTMLVideoElement>(null);
+  const [error, setError] = useState<string>();
+  const doneRef = useRef(false);
+
+  useEffect(() => {
+    let stream: MediaStream | undefined;
+    let raf = 0;
+    const canvas = document.createElement('canvas');
+    const ctx = canvas.getContext('2d', { willReadFrequently: true });
+
+    const tick = () => {
+      const video = videoRef.current;
+      if (!doneRef.current && video && ctx && video.readyState === video.HAVE_ENOUGH_DATA) {
+        canvas.width = video.videoWidth;
+        canvas.height = video.videoHeight;
+        ctx.drawImage(video, 0, 0, canvas.width, canvas.height);
+        const image = ctx.getImageData(0, 0, canvas.width, canvas.height);
+        const result = jsQR(image.data, image.width, image.height);
+        if (result && result.binaryData.length > 0) {
+          doneRef.current = true;
+          onScan(new Uint8ClampedArray(result.binaryData));
+          return;
+        }
+      }
+      raf = requestAnimationFrame(tick);
+    };
+
+    (async () => {
+      try {
+        stream = await navigator.mediaDevices.getUserMedia({
+          video: { facingMode: 'environment' },
+        });
+        if (videoRef.current) {
+          videoRef.current.srcObject = stream;
+          await videoRef.current.play();
+        }
+        raf = requestAnimationFrame(tick);
+      } catch {
+        setError(
+          'Could not access the camera. Grant camera permission, or verify with emojis instead.',
+        );
+      }
+    })();
+
+    return () => {
+      doneRef.current = true;
+      cancelAnimationFrame(raf);
+      stream?.getTracks().forEach((track) => track.stop());
+    };
+  }, [onScan]);
+
+  if (error) {
+    return (
+      <Box direction="Column" gap="400">
+        <Text style={{ color: color.Critical.Main }} size="T300">
+          {error}
+        </Text>
+        <Button variant="Secondary" fill="Soft" onClick={onCancel}>
+          <Text size="B400">Back</Text>
+        </Button>
+      </Box>
+    );
+  }
+
+  return (
+    <Box direction="Column" gap="400" alignItems="Center">
+      <Text size="T300" align="Center">
+        Point your camera at the QR code shown on your other device.
+      </Text>
+      <video
+        ref={videoRef}
+        muted
+        playsInline
+        style={{
+          width: '100%',
+          maxWidth: 280,
+          borderRadius: config.radii.R400,
+          background: '#000',
+        }}
+      >
+        <track kind="captions" />
+      </video>
+      <Button variant="Secondary" fill="Soft" onClick={onCancel}>
+        <Text size="B400">Cancel</Text>
+      </Button>
+    </Box>
+  );
+}