fix(api): add ttl_seconds to QR challenge response and fix client-side countdown

The QR login page countdown timer compared the server's UTC expiration
timestamp against the client's local clock, causing the QR code to appear
immediately expired when the client clock was ahead of the server.

Changes:
- Add ttl_seconds field to CreateChallengeResponse (seconds until expiry)
- Frontend countdown now uses relative elapsed time since response was
  received, eliminating clock-skew issues
- Mobile app: replace alert-only QR button with actual camera-based
  QR code scanner using expo-camera
- Add QRScannerScreen with barcode scanning, permission handling, and
  scan area overlay
- Update camera permission description to mention QR code scanning
- Add tests for ttl_seconds computation

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/templates/qr_login.html

@@ -126,6 +126,8 @@ function qrLoginPage() {
     errorMsg: '',
     _pollTimer: null,
     _countdownTimer: null,
+    _ttlSeconds: 0,
+    _receivedAt: null,
 
     _csrfToken() {
       return document.cookie
@@ -156,6 +158,8 @@ function qrLoginPage() {
         this.challengeToken = data.challenge_token;
         this.qrPayload = data.qr_payload;
         this.expiresAt = new Date(data.expires_at);
+        this._ttlSeconds = data.ttl_seconds || 120;
+        this._receivedAt = Date.now();
         this.status = 'pending';
         this.deviceName = '';
 
@@ -214,8 +218,9 @@ function qrLoginPage() {
     },
 
     _updateCountdown() {
-      if (!this.expiresAt) { this.countdown = 0; return; }
-      const remaining = Math.max(0, Math.floor((this.expiresAt - new Date()) / 1000));
+      if (!this._receivedAt) { this.countdown = 0; return; }
+      const elapsed = (Date.now() - this._receivedAt) / 1000;
+      const remaining = Math.max(0, Math.floor(this._ttlSeconds - elapsed));
       this.countdown = remaining;
     },