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

lib/view/login/login_controller.dart

@@ -3,6 +3,7 @@ import 'dart:developer';
 
 import 'package:flutter/foundation.dart';
 
+import '../../api/demo/demo_mode.dart';
 import '../../api/errors/auth_exception.dart';
 import '../../api/errors/error_mapper.dart';
 import '../../api/marianumconnect/auth/device_token_name.dart';
@@ -33,6 +34,20 @@ class LoginController extends ChangeNotifier {
     notifyListeners();
 
     final user = username.trim().toLowerCase();
+
+    // Demo login: the prefix enters local demo mode, password ignored, no
+    // network (see DemoMode).
+    if (DemoMode.matches(user)) {
+      await AccountData().removeData();
+      await const MarianumConnectTokenStorage().clear();
+      await WidgetSync.clear();
+      await WidgetSync.triggerUpdate();
+      await AccountData().setDemo(user);
+      _loading = false;
+      notifyListeners();
+      return true;
+    }
+
     try {
       await AccountData().removeData();
       // Vorherigen Token revoken bevor wir einen neuen anfordern — ein altes