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

lib/view/pages/talk/widgets/chat_bubble.dart

@@ -9,6 +9,7 @@ import '../../../../share_intent/remote_file_ref.dart';
 import '../../../../state/app/modules/chat/bloc/chat_bloc.dart';
 import '../../../../utils/downloads/download_job.dart';
 import '../../../../utils/haptics.dart';
+import '../../../../widget/demo_restricted.dart';
 import '../../../../widget/downloads/download_trigger.dart';
 import '../data/chat_bubble_styles.dart';
 import '../data/chat_message.dart';
@@ -177,6 +178,7 @@ class _ChatBubbleState extends State<ChatBubble>
       return;
     }
     if (message.file == null) return;
+    if (guardDemoAction(context)) return;
     if (isDownloading) {
       confirmCancelDownload();
     } else {