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/web/firebase-messaging-sw.js

@@ -1,5 +1,7 @@
 // Background handler for FCM web push. Keep the config in sync with
 // window.KC_FIREBASE in index.html (a service worker can't read window).
+// Analytics is NOT initialised here — it only makes sense on visible pages
+// with a real navigator context; the main index.html handles it.
 importScripts('https://www.gstatic.com/firebasejs/10.13.2/firebase-app-compat.js');
 importScripts('https://www.gstatic.com/firebasejs/10.13.2/firebase-messaging-compat.js');