docs: push notifications module + FCM client wiring

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

client/app/README.md

@@ -59,6 +59,11 @@ Teamer login only).
   (`verantwortliche_register_screen.dart`) — shown on the home screen to a
   logged-in Authentik user without a membership: enter a KC invite code,
   pick a Gemeinde, submit; a Leitungsteam member then approves.
+- **Push (web)** — `web/index.html` loads the Firebase compat SDK and
+  `web/firebase-messaging-sw.js` handles background messages. After login
+  `AppState` calls `window.kcGetPushToken()` and registers the token
+  (`POST /push/register`). Inert until `apiKey` / `appId` / `vapidKey` are
+  filled into both files (see the `REPLACE_ME` placeholders).
 - **Workshop-Wahl** (`lib/screens/wahl_screen.dart`, guests) — two tabs:
   *Wünsche* (`GET /wahl/guest/overview`, tap workshops in order, max 3,
   `POST /wahl/:id/teilnehmer`) and *Ergebnis* (`GET /wahl/guest/results` —