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

lib/api/marianumconnect/auth/session_validator.dart

@@ -14,6 +14,9 @@ class SessionValidator {
     required Future<void> Function() onInvalidated,
   }) async {
     if (!AccountData().isPopulated()) return;
+    // AuthVerify uses its own dio (bypassing the demo interceptor), so a demo
+    // session must be skipped here or its missing token would 401 into a logout.
+    if (AccountData().isDemo) return;
     final username = AccountData().getUsername();
     final password = AccountData().getPassword();
     try {