implemented a comprehensive client-side demo mode triggered by a "demo@" username prefix, serving curated fixture data for all app modules

lib/push/push_registration.dart

@@ -5,6 +5,7 @@ import 'package:firebase_messaging/firebase_messaging.dart';
 import 'package:nextcloud/notifications.dart' show generatePushTokenHash;
 import 'package:package_info_plus/package_info_plus.dart';
 
+import '../api/demo/demo_mode.dart';
 import '../api/marianumcloud/app_password/delete_app_password.dart';
 import '../api/marianumcloud/app_password/get_app_password.dart';
 import '../api/marianumconnect/marianumconnect_endpoint.dart';
@@ -93,6 +94,7 @@ class PushRegistration {
   /// transport failures are logged and swallowed so callers can
   /// fire-and-forget (and simply ignore the result).
   Future<bool> register() async {
+    if (DemoMode.active) return false;
     final String? fcmToken;
     try {
       fcmToken = await FirebaseMessaging.instance.getToken();
@@ -383,6 +385,7 @@ class PushRegistration {
   /// is called with), then clear them locally. Ordered so the proxy stops
   /// pushing before credentials are gone.
   Future<void> logoutCleanup() async {
+    if (DemoMode.active) return;
     await unregister();
     try {
       await DeleteAppPassword().run();