fix(app): say it out loud when wake-ups do not work

Registration failures from the distributor were swallowed, so push looked
enabled while nothing arrived. Without wake-ups Android suspends the app
and messages only show up once the user opens it — which looks like broken
delivery even though the relay has them queued.

The failure is now remembered and surfaced in Settings, and the chat list
carries a dismissible line explaining the state.

app/lib/core/push/push_service.dart

@@ -144,6 +144,10 @@ class PushService {
     return (await UnifiedPush.getDistributor()) != null ? 'up' : 'off';
   }
 
+  /// Odmítl distributor poslední registraci? Nastavení to ukáže — jinak by
+  /// push vypadal zapnutý, ale budíčky by nechodily.
+  Future<bool> registrationFailed() => PushBootstrap.registrationFailed();
+
   Future<void> setModeOff() async {
     await _teardownFcm(deleteToken: true);
     try {