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>

package.json

@@ -74,6 +74,7 @@
     "immer": "11.1.8",
     "is-hotkey": "0.2.0",
     "jotai": "2.20.0",
+    "jsqr": "1.4.0",
     "katex": "0.16.11",
     "linkify-react": "4.3.3",
     "linkifyjs": "4.3.3",
@@ -82,6 +83,7 @@
     "millify": "6.1.0",
     "pdfjs-dist": "5.7.284",
     "prismjs": "1.30.0",
+    "qrcode": "1.5.4",
     "qrcode.react": "4.2.0",
     "react": "19.2.6",
     "react-aria": "3.48.0",
@@ -112,6 +114,7 @@
     "@types/katex": "0.16.8",
     "@types/node": "25.9.1",
     "@types/prismjs": "1.26.6",
+    "@types/qrcode": "1.5.6",
     "@types/react": "19.2.15",
     "@types/react-dom": "19.2.3",
     "@types/react-google-recaptcha": "2.1.9",