Add Auth service and Login Page

frontend_splatournament_manager/lib/providers/auth_provider.dart

@@ -0,0 +1,74 @@
+import 'package:flutter/material.dart';
+import 'package:frontend_splatournament_manager/services/auth_service.dart';
+
+class AuthProvider extends ChangeNotifier {
+  final AuthService _authService = AuthService();
+
+  bool _isLoggedIn = false;
+  String? _username;
+  int? _userId;
+  String? _error;
+  bool _isLoading = false;
+
+  bool get isLoggedIn => _isLoggedIn;
+  String? get username => _username;
+  int? get userId => _userId;
+  String? get error => _error;
+  bool get isLoading => _isLoading;
+
+  Future<bool> login(String username, String password) async {
+    _isLoading = true;
+    _error = null;
+    notifyListeners();
+
+    try {
+      final user = await _authService.login(username, password);
+      _isLoggedIn = true;
+      _username = user['username'];
+      _userId = user['id'];
+      _isLoading = false;
+      notifyListeners();
+      return true;
+    } catch (e) {
+      _error = e.toString().replaceFirst('Exception: ', '');
+      _isLoading = false;
+      notifyListeners();
+      return false;
+    }
+  }
+
+  Future<bool> register(String username, String password) async {
+    _isLoading = true;
+    _error = null;
+    notifyListeners();
+
+    try {
+      final user = await _authService.register(username, password);
+      _isLoggedIn = true;
+      _username = user['username'];
+      _userId = user['id'];
+      _isLoading = false;
+      notifyListeners();
+      return true;
+    } catch (e) {
+      _error = e.toString().replaceFirst('Exception: ', '');
+      _isLoading = false;
+      notifyListeners();
+      return false;
+    }
+  }
+
+  void logout() {
+    _isLoggedIn = false;
+    _username = null;
+    _userId = null;
+    _error = null;
+    notifyListeners();
+  }
+
+  void clearError() {
+    _error = null;
+    notifyListeners();
+  }
+}
+