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

lib/widget/demo_restricted.dart

@@ -0,0 +1,21 @@
+import 'package:flutter/widgets.dart';
+
+import '../api/demo/demo_mode.dart';
+import 'info_dialog.dart';
+
+/// Guards an action the demo mode can't back with real data (writes, uploads,
+/// downloads, user/element lookups). Shows an info dialog and returns `true`
+/// when a demo session is active — callers abort on `true`:
+///
+/// ```dart
+/// if (guardDemoAction(context)) return;
+/// ```
+bool guardDemoAction(BuildContext context) {
+  if (!DemoMode.active) return false;
+  InfoDialog.show(
+    context,
+    'Diese Funktion steht im Demo-Modus nicht zur Verfügung.',
+    title: 'Demo-Modus',
+  );
+  return true;
+}