Multiple improvements in authentication functionality

frontend/lib/user_checker.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
 import 'backend.dart';
 import 'login_screen.dart';
 import 'chat_screen.dart';
@@ -11,12 +12,26 @@ class UserChecker extends StatefulWidget {
 
 class _UserCheckerState extends State<UserChecker> {
   String? username;
-  bool isLoading = false;
+  bool isLoading = true;
+  final _storage = const FlutterSecureStorage();
 
   @override
   void initState() {
     super.initState();
-    username = null;
+    _checkSavedCredentials();
+  }
+
+  Future<void> _checkSavedCredentials() async {
+    final savedUsername = await _storage.read(key: 'username');
+    final savedPassword = await _storage.read(key: 'password');
+
+    if (savedUsername != null && savedPassword != null) {
+      _handleLogin(savedUsername, savedPassword);
+    } else {
+      setState(() {
+        isLoading = false;
+      });
+    }
   }
 
   static void fetchChats(Function(List<int>) onSuccess) {
@@ -28,13 +43,15 @@ class _UserCheckerState extends State<UserChecker> {
     });
   }
 
-  void _handleLogin(String user, String password) {
-    Backend.auth(user, password);
-  
+  void _handleLogin(String user, String password) async {
     setState(() {
       username = user;
-      isLoading = true; // Show loading spinner
+      isLoading = true;
     });
+  
+    await Backend.auth(user, password);
+    await _storage.write(key: 'username', value: user);
+    await _storage.write(key: 'password', value: password);
 
     fetchChats((chatIds) {
       if (chatIds.isEmpty) {
@@ -49,14 +66,13 @@ class _UserCheckerState extends State<UserChecker> {
         );
       }
     });
-
   }
 
   @override
   Widget build(BuildContext context) {
     print("Build UserChecker: $username $isLoading");
     if (isLoading) {
-      return Center(child: CircularProgressIndicator()); // Loading state UI
+      return Center(child: CircularProgressIndicator());
     }
 
     return LoginScreen(onLogin: _handleLogin);