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/api.dart

@@ -450,8 +450,14 @@ class Api {
     return j['accessToken'] as String;
   }
 
-  Future<String> teamLogin(String email, String password) async {
-    final j = await _post('/auth/team-login', {'email': email, 'password': password});
+  /// Logs a Gemeinde Teamer in by Gemeinde name (the normal path) or by
+  /// email (legacy/personal accounts) — pass exactly one of the two.
+  Future<String> teamLogin({String? gemeindeName, String? email, required String password}) async {
+    final j = await _post('/auth/team-login', {
+      if (gemeindeName != null && gemeindeName.isNotEmpty) 'gemeindeName': gemeindeName,
+      if (email != null && email.isNotEmpty) 'email': email,
+      'password': password,
+    });
     return j['accessToken'] as String;
   }
 
@@ -772,8 +778,8 @@ class AppState extends ChangeNotifier {
   Future<void> guestLogin(String code, String first, String last) =>
       _api.guestLogin(code, first, last).then(_establish);
 
-  Future<void> teamLogin(String email, String password) =>
-      _api.teamLogin(email, password).then(_establish);
+  Future<void> teamLogin({String? gemeindeName, String? email, required String password}) =>
+      _api.teamLogin(gemeindeName: gemeindeName, email: email, password: password).then(_establish);
 
   Future<void> redeemInvite({
     required String token,