feat: add auth screens and state management

lib/auth/auth_state.dart

@@ -0,0 +1,18 @@
+import 'package:flutter/material.dart';
+import 'package:firebase_auth/firebase_auth.dart';
+
+class AuthState extends ChangeNotifier {
+  User? _user;
+  User? get user => _user;
+
+  final FirebaseAuth _auth = FirebaseAuth.instance;
+
+  AuthState() {
+    _auth.authStateChanges().listen((user) {
+      _user = user;
+      notifyListeners();
+    });
+  }
+
+  bool get isLoggedIn => _user != null;
+}