feat(backend): Wahl-Phasen, Verantwortliche-Invites, Auth fixes

- New VerantwortlicheInvite model: LT-issued invites so a person can
  register as Gemeinde Verantwortliche(r) for a specific Gemeinde,
  skipping the self-registration approval step.
- Wahl/Workshop/Teilnehmer gain phase support (phasenAnzahl,
  beschreibung), mirroring the WP plugin's multi-phase elections.
  Teilnehmer unique constraint now scoped per phase.
- Auth: team login + guest auth adjustments, spec coverage.
- sync.service.ts: register VerantwortlicheInvite as a synced model.
- wahl.service.ts: submitTeilnehmer updated for the new phase-scoped
  unique key.
- client: login/home screen rework, new theme.dart, FCM web tweaks.
- .gitignore: ignore .DS_Store.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

client/app/lib/main.dart

@@ -4,6 +4,7 @@ import 'package:http/http.dart' as http;
 import 'api.dart';
 import 'screens/home_screen.dart';
 import 'screens/login_screen.dart';
+import 'theme.dart';
 
 void main() {
   final state = AppState(Api(http.Client()))..bootstrap();
@@ -34,10 +35,7 @@ class KcApp extends StatelessWidget {
       child: MaterialApp(
         title: 'KC-App',
         debugShowCheckedModeBanner: false,
-        theme: ThemeData(
-          colorSchemeSeed: const Color(0xFF3B5BA5),
-          useMaterial3: true,
-        ),
+        theme: buildKcTheme(),
         home: const _AuthGate(),
       ),
     );