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;

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

@@ -1,61 +1,12 @@
 import 'package:flutter_bloc/flutter_bloc.dart';
 import 'package:formz/formz.dart';
 import 'package:equatable/equatable.dart';
+import '../../../../core/api/api_exception.dart';
+import '../../../../core/form_inputs/form_inputs.dart';
 import '../../data/auth_repository.dart';
 import '../../data/models/signup_request.dart';
 import '../../data/models/auth_response.dart';
 
-class Username extends FormzInput<String, String> {
-  const Username.pure() : super.pure('');
-  const Username.dirty([super.value = '']) : super.dirty();
-
-  @override
-  String? validator(String value) {
-    if (value.isEmpty) return 'Username is required';
-    if (value.length < 3) return 'Username must be at least 3 characters';
-    if (value.length > 30) return 'Username must be at most 30 characters';
-    return null;
-  }
-}
-
-class Email extends FormzInput<String, String> {
-  const Email.pure() : super.pure('');
-  const Email.dirty([super.value = '']) : super.dirty();
-
-  static final _regex = RegExp(r'^[\w.-]+@[\w.-]+\.\w+$');
-
-  @override
-  String? validator(String value) {
-    if (value.isEmpty) return 'Email is required';
-    if (!_regex.hasMatch(value)) return 'Invalid email format';
-    return null;
-  }
-}
-
-class Password extends FormzInput<String, String> {
-  const Password.pure() : super.pure('');
-  const Password.dirty([super.value = '']) : super.dirty();
-
-  @override
-  String? validator(String value) {
-    if (value.isEmpty) return 'Password is required';
-    if (value.length < 6) return 'Password must be at least 6 characters';
-    return null;
-  }
-}
-
-class VerificationCode extends FormzInput<String, String> {
-  const VerificationCode.pure() : super.pure('');
-  const VerificationCode.dirty([super.value = '']) : super.dirty();
-
-  @override
-  String? validator(String value) {
-    if (value.isEmpty) return 'Code is required';
-    if (!RegExp(r'^\d{6}$').hasMatch(value)) return 'Code must be 6 digits';
-    return null;
-  }
-}
-
 class RegisterState extends Equatable {
   final Username username;
   final Email email;
@@ -159,10 +110,11 @@ class RegisterCubit extends Cubit<RegisterState> {
       );
       return true;
     } catch (e) {
+      final message = e is ApiException ? e.message : e.toString();
       emit(
         state.copyWith(
           status: FormzSubmissionStatus.failure,
-          errorMessage: e.toString(),
+          errorMessage: message,
         ),
       );
       return false;
@@ -184,10 +136,11 @@ class RegisterCubit extends Cubit<RegisterState> {
       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;
@@ -203,7 +156,8 @@ class RegisterCubit extends Cubit<RegisterState> {
       );
       emit(state.copyWith(codeSent: true));
     } catch (e) {
-      emit(state.copyWith(errorMessage: e.toString()));
+      final message = e is ApiException ? e.message : e.toString();
+      emit(state.copyWith(errorMessage: message));
     }
   }
 }