fix(apps): consolidate FormzInput validators and fix login screen

- Move FormzInput validators to core/form_inputs/form_inputs.dart
- Fix login_screen.dart syntax error (missing 'class' keyword)
- Remove unused _isLoading field
- Fix unnecessary const keywords
- Update login_cubit and register_cubit imports
- Remove duplicate FormzInput definitions from register_cubit
- Add Toast and Banner UI feedback system
- Remove legacy login/register screens (login_code, login_email, login_password, register_step2)
- Remove unused warning_banner widget
- Update tests for new error messages and DI setup

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

@@ -1,10 +1,11 @@
 import 'package:flutter_bloc/flutter_bloc.dart';
 import 'package:formz/formz.dart';
 import 'package:equatable/equatable.dart';
+import '../../../../core/api/api_exception.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;
+import '../../../../core/form_inputs/form_inputs.dart';
 
 class LoginState extends Equatable {
   final Email email;
@@ -64,10 +65,11 @@ class LoginCubit extends Cubit<LoginState> {
       emit(state.copyWith(status: FormzSubmissionStatus.success));
       return response;
     } catch (e) {
+      final message = e is ApiException ? e.message : e.toString();
       emit(
         state.copyWith(
           status: FormzSubmissionStatus.failure,
-          errorMessage: e.toString(),
+          errorMessage: message,
         ),
       );
       return null;