fix(admin): fix greyed-out QR login toggle on admin connections page

- Add `qr_login_enabled` boolean config field (default True) to app/config.py
- Add `qr_login_enabled` entry to settings metadata in app/utils/settings_service.py
- Fix app/views/settings.py to use `qr_login_enabled` directly instead of
  deriving enablement status from qr_login_challenge_ttl_seconds (integer TTL)
- Fix admin_connections.html: remove hardcoded `disabled` attribute from the
  Mobile Phone Upload toggle and wire up onchange handler so toggling actually
  persists the setting via toggleSetting('qr_login_enabled', this.checked)
- Gate all three QR auth API endpoints on settings.qr_login_enabled so the
  feature is actually disabled when the toggle is turned off

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/DocuElevate/sessions/8421cdb2-d92d-4b83-9cda-c44983d35173

app/config.py

@@ -244,6 +244,10 @@ class Settings(BaseSettings):
             "Useful for admin-configured non-standard durations."
         ),
     )
+    qr_login_enabled: bool = Field(
+        default=True,
+        description="Enable QR code-based login for mobile device authentication (default: True).",
+    )
     qr_login_challenge_ttl_seconds: int = Field(
         default=120,
         description="Time-to-live in seconds for QR login challenges (default: 2 minutes).",