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

lib/model/account_data.dart

@@ -18,6 +18,10 @@ class AccountData {
   // token, so two registrations need two app passwords.
   static const _appPasswordField = 'nextcloud_app_password';
   static const _appPasswordTalkField = 'nextcloud_app_password_talk';
+  // Persists the demo session across cold starts (see DemoMode).
+  static const _demoField = 'is_demo';
+  // Keeps isPopulated()/getPassword() valid; demo mode never uses a real one.
+  static const _demoPasswordPlaceholder = 'demo';
 
   static const FlutterSecureStorage _secureStorage = FlutterSecureStorage();
 
@@ -34,6 +38,10 @@ class AccountData {
   String? _password;
   String? _appPassword;
   String? _appPasswordTalk;
+  bool _isDemo = false;
+
+  /// True while the active session is a local demo session (see DemoMode).
+  bool get isDemo => _isDemo;
 
   String getUsername() {
     if (_username == null) throw Exception('Username not initialized');
@@ -65,14 +73,31 @@ class AccountData {
     if (!_populated.isCompleted) _populated.complete();
   }
 
+  /// Enters a local demo session for [username] — no real credentials or token;
+  /// every backend is served from fixtures while [isDemo] is true (see DemoMode).
+  Future<void> setDemo(String username) async {
+    await _secureStorage.write(key: _usernameField, value: username);
+    await _secureStorage.write(
+      key: _passwordField,
+      value: _demoPasswordPlaceholder,
+    );
+    await _secureStorage.write(key: _demoField, value: 'true');
+    _username = username;
+    _password = _demoPasswordPlaceholder;
+    _isDemo = true;
+    if (!_populated.isCompleted) _populated.complete();
+  }
+
   Future<void> removeData() async {
     _populated = Completer();
     _username = null;
     _password = null;
     _appPassword = null;
     _appPasswordTalk = null;
+    _isDemo = false;
     await _secureStorage.delete(key: _usernameField);
     await _secureStorage.delete(key: _passwordField);
+    await _secureStorage.delete(key: _demoField);
     await _clearAppPasswordStorage();
     await _clearAppPasswordTalkStorage();
   }
@@ -139,6 +164,7 @@ class AccountData {
     await _migrateFromLegacyStorage();
     _username = await _secureStorage.read(key: _usernameField);
     _password = await _secureStorage.read(key: _passwordField);
+    _isDemo = (await _secureStorage.read(key: _demoField)) == 'true';
     try {
       _appPassword = await pushSecureStorage.read(key: _appPasswordField);
       _appPasswordTalk = await pushSecureStorage.read(