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>

LOTUS_TESTING.md

@@ -675,6 +675,8 @@ Run the axe DevTools extension (or Lighthouse → Accessibility) on a room view,
 
 ## Outstanding verification backlog
 
+**QR Device Verification (2026-07):** With two logged-in Lotus sessions (or Lotus + Element), start a device verification. On the **Ready** step you now see your own QR code plus a **"Scan their QR code"** button and a **"Verify with emoji instead"** fallback. Have one device **scan** the other's code (grant camera permission) → the showing device asks you to **Confirm**, and both reach **verified**. Check: emoji-SAS still works unchanged; denying camera shows a graceful "verify with emojis instead" message; a deliberately-wrong scan cancels cleanly. Desktop (WebView2) auto-grants the camera; web needs the Permissions-Policy camera allowance (already set).
+
 **Disappearing Messages (MSC1763 `m.room.retention`, 2026-07):** In Room Settings → General → **Message Retention**, an admin picks Off / 1 Day / 1 Week / 1 Month (non-admins see the buttons disabled). After setting e.g. 1 Day, messages older than a day **vanish from the timeline** for everyone in Lotus (toggle Settings → General → **Show Hidden Events** to reveal them again). Setting back to **Off** restores them. Separately, each user can enable Settings → General → **Enforce Message Retention** (default OFF) → their OWN expired messages then get **permanently redacted** within ~30 s (verify: OTHER people's messages are NEVER redacted by this; only your own). Note true server-side purge also needs Synapse `retention:` configured.
 
 **Mark as Unread + Low Priority (MSC2867 / m.lowpriority, 2026-07):** Right-click a room in the sidebar → **Mark as Unread** puts a dot on the row (bold name) even with no new messages; opening/reading the room clears it, and it syncs to another device. **Mark as Read** on a marked room clears it too. Right-click → **Add to Low Priority** moves the room into a collapsed "Low Priority" category at the bottom of the room list (and removes it from Favorites if it was there, and vice-versa); **Remove from Low Priority** returns it to Rooms.