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>

app/api/qr_auth.py

@@ -70,6 +70,7 @@ class CreateChallengeResponse(BaseModel):
     challenge_id: int
     challenge_token: str
     expires_at: datetime
+    ttl_seconds: int = Field(description="Seconds until the challenge expires (use for client-side countdown).")
     qr_payload: str = Field(description="The string to encode in the QR code.")
 
 
@@ -131,10 +132,16 @@ async def create_challenge(
     base_url = str(request.base_url).rstrip("/")
     qr_payload = f"docuelevate://qr-login?token={challenge.challenge_token}&server={base_url}"
 
+    # Compute the TTL in seconds so the client can run a countdown timer
+    # without comparing absolute timestamps (which breaks when client and
+    # server clocks are out of sync).
+    ttl_seconds = max(0, int((challenge.expires_at - challenge.created_at).total_seconds()))
+
     return {
         "challenge_id": challenge.id,
         "challenge_token": challenge.challenge_token,
         "expires_at": challenge.expires_at,
+        "ttl_seconds": ttl_seconds,
         "qr_payload": qr_payload,
     }