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

frontend/templates/admin_connections.html

@@ -46,7 +46,8 @@
       <div>
         <label class="relative inline-flex items-center cursor-pointer">
           <input type="checkbox" id="qr-upload-toggle" class="sr-only peer"
-                 {% if qr_login_enabled %}checked{% endif %} disabled>
+                 {% if qr_login_enabled %}checked{% endif %}
+                 onchange="toggleSetting('qr_login_enabled', this.checked)">
           <div class="w-11 h-6 bg-gray-200 peer-focus:outline-none peer-focus:ring-2 peer-focus:ring-indigo-500 rounded-full peer dark:bg-gray-700 peer-checked:after:translate-x-full peer-checked:after:border-white after:content-[''] after:absolute after:top-[2px] after:left-[2px] after:bg-white after:border-gray-300 after:border after:rounded-full after:h-5 after:w-5 after:transition-all dark:border-gray-600 peer-checked:bg-indigo-600" style="min-width:44px; min-height:24px;"></div>
           <span class="ml-3 text-sm font-medium text-gray-700 dark:text-gray-300">{{ _("connections.qr_code_enabled") }}</span>
         </label>