Move backend config to a dialog; group settings into cards

Settings screen now splits responsibilities:
- Backend URL + API key move into a Test/Save popup dialog
  (backend_config_dialog.dart), reachable via a "Re-configure" action.
  Per M3 the Test button is left-aligned (neutral) with Cancel/Save
  grouped trailing. On first run the dialog auto-opens non-dismissible.
- The screen shows the connection read-only (URL plain, key masked with
  reveal) in a "Backend connection" card, and groups Theme + Push into an
  "App settings" card. Theme and push apply immediately, no Save button.
- Align all card titles to titleLarge across home/status/settings.

Adds a test seam (dioBuilderForTesting) so reconfigure keeps the mock Dio
in widget tests instead of issuing real requests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/test/helpers/backend_test_harness.dart

@@ -63,5 +63,8 @@ Future<DioAdapter> setUpBackendForTest({
   final dio = Dio(BaseOptions(baseUrl: 'http://test.local/api'));
   final adapter = DioAdapter(dio: dio);
   BackendAPI.instance.dioForTesting = dio;
+  // Keep the mock client even after a reconfigure (e.g. saving the backend
+  // settings) so no test ever issues a real network request.
+  BackendAPI.dioBuilderForTesting = (_, _) => dio;
   return adapter;
 }