refactor entire frontend

frontend/lib/user_checker.dart

@@ -0,0 +1,64 @@
+import 'package:flutter/material.dart';
+import 'backend.dart';
+import 'login_screen.dart';
+import 'chat_screen.dart';
+import 'chat_list_screen.dart';
+
+class UserChecker extends StatefulWidget {
+  @override
+  _UserCheckerState createState() => _UserCheckerState();
+}
+
+class _UserCheckerState extends State<UserChecker> {
+  String? username;
+  bool isLoading = false;
+
+  @override
+  void initState() {
+    super.initState();
+    username = null;
+  }
+
+  static void fetchChats(Function(List<int>) onSuccess) {
+    Backend.sendAction("get-chats", {}).then((response) {
+      print("chats response: $response");
+      List<dynamic> chatObjects = response['chats'];
+      List<int> chatIds = chatObjects.map((chat) => chat['id'] as int).toList();
+      onSuccess(chatIds);
+    });
+  }
+
+  void _handleLogin(String user, String password) {
+    Backend.auth(user, password);
+  
+    setState(() {
+      username = user;
+      isLoading = true; // Show loading spinner
+    });
+
+    fetchChats((chatIds) {
+      if (chatIds.isEmpty) {
+        Navigator.pushReplacement(
+          context,
+          MaterialPageRoute(builder: (_) => ChatListScreen()),
+        );
+      } else {
+        Navigator.pushReplacement(
+          context,
+          MaterialPageRoute(builder: (_) => ChatScreen(chatId: chatIds.first)),
+        );
+      }
+    });
+
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    print("Build UserChecker: $username $isLoading");
+    if (isLoading) {
+      return Center(child: CircularProgressIndicator()); // Loading state UI
+    }
+
+    return LoginScreen(onLogin: _handleLogin);
+  }
+}