feat(apps): add LoginCubit for login form

apps/lib/features/auth/presentation/cubits/login_cubit.dart

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+import 'package:flutter_bloc/flutter_bloc.dart';
+import 'package:formz/formz.dart';
+import 'package:equatable/equatable.dart';
+import '../../data/auth_repository.dart';
+import '../../data/models/login_request.dart';
+import '../../data/models/auth_response.dart';
+import 'register_cubit.dart' show Email, Password;
+
+class LoginState extends Equatable {
+  final Email email;
+  final Password password;
+  final FormzSubmissionStatus status;
+  final String? errorMessage;
+
+  const LoginState({
+    this.email = const Email.pure(),
+    this.password = const Password.pure(),
+    this.status = FormzSubmissionStatus.initial,
+    this.errorMessage,
+  });
+
+  bool get isValid => email.isValid && password.isValid;
+
+  LoginState copyWith({
+    Email? email,
+    Password? password,
+    FormzSubmissionStatus? status,
+    String? errorMessage,
+  }) {
+    return LoginState(
+      email: email ?? this.email,
+      password: password ?? this.password,
+      status: status ?? this.status,
+      errorMessage: errorMessage,
+    );
+  }
+
+  @override
+  List<Object?> get props => [email, password, status, errorMessage];
+}
+
+class LoginCubit extends Cubit<LoginState> {
+  final AuthRepository _repository;
+
+  LoginCubit(this._repository) : super(const LoginState());
+
+  void emailChanged(String value) {
+    emit(state.copyWith(email: Email.dirty(value)));
+  }
+
+  void passwordChanged(String value) {
+    emit(state.copyWith(password: Password.dirty(value)));
+  }
+
+  Future<AuthResponse?> submit() async {
+    if (!state.isValid) return null;
+
+    emit(state.copyWith(status: FormzSubmissionStatus.inProgress));
+
+    try {
+      final response = await _repository.login(
+        LoginRequest(email: state.email.value, password: state.password.value),
+      );
+      emit(state.copyWith(status: FormzSubmissionStatus.success));
+      return response;
+    } catch (e) {
+      emit(
+        state.copyWith(
+          status: FormzSubmissionStatus.failure,
+          errorMessage: e.toString(),
+        ),
+      );
+      return null;
+    }
+  }
+}

apps/test/features/auth/presentation/cubits/login_cubit_test.dart

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+import 'package:bloc_test/bloc_test.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:formz/formz.dart';
+import 'package:mocktail/mocktail.dart';
+import 'package:social_app/features/auth/data/auth_repository.dart';
+import 'package:social_app/features/auth/presentation/cubits/login_cubit.dart';
+
+class MockAuthRepository extends Mock implements AuthRepository {}
+
+void main() {
+  late LoginCubit cubit;
+  late MockAuthRepository mockRepository;
+
+  setUp(() {
+    mockRepository = MockAuthRepository();
+    cubit = LoginCubit(mockRepository);
+  });
+
+  tearDown(() {
+    cubit.close();
+  });
+
+  group('LoginCubit', () {
+    test('initial state has pure status', () {
+      expect(cubit.state.status, FormzSubmissionStatus.initial);
+    });
+
+    blocTest<LoginCubit, LoginState>(
+      'emailChanged updates email',
+      build: () => cubit,
+      act: (c) => c.emailChanged('test@example.com'),
+      expect: () => [isA<LoginState>()],
+    );
+
+    blocTest<LoginCubit, LoginState>(
+      'passwordChanged updates password',
+      build: () => cubit,
+      act: (c) => c.passwordChanged('password123'),
+      expect: () => [isA<LoginState>()],
+    );
+  });
+}