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/lib/widgets/scanner_status_card.dart

@@ -95,7 +95,7 @@ class _ScannerStatusCardState<T> extends State<ScannerStatusCard<T>>
                           children: [
                             Text(
                               widget.title,
-                              style: theme.textTheme.titleMedium,
+                              style: theme.textTheme.titleLarge,
                             ),
                             if (isStale) ...[
                               const SizedBox(width: 6),