Multiple improvements in authentication functionality

frontend/lib/app_menu.dart

@@ -1,5 +1,8 @@
 import 'package:flutter/material.dart';
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
 import 'chat_list_screen.dart';
+import 'user_checker.dart';
+import 'backend.dart';
 
 class AppMenu extends StatelessWidget {
   const AppMenu({super.key});
@@ -21,6 +24,22 @@ class AppMenu extends StatelessWidget {
               );
             },
           ),
+          ListTile(
+            title: const Text('Logout'),
+            onTap: () async {
+              final storage = const FlutterSecureStorage();
+              await storage.delete(key: 'username');
+              await storage.delete(key: 'password');
+              Backend.disconnect();
+
+              Navigator.push(
+                context,
+                MaterialPageRoute(
+                  builder: (context) => UserChecker(),
+                ),
+              );
+            },
+          ),
           // Add other settings options here
         ],
       ),