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/core/api/api_client.dart

@@ -6,36 +6,42 @@ import '../storage/token_storage.dart';
 class ApiClient {
   final Dio _dio;
   final TokenStorage _tokenStorage;
-  final Future<bool> Function(String)? _refreshToken;
+  final ApiInterceptor _interceptor;
 
-  ApiClient({
+  factory ApiClient({
     required String baseUrl,
     required TokenStorage tokenStorage,
     Dio? dio,
-    Future<bool> Function(String)? refreshToken,
-  }) : _tokenStorage = tokenStorage,
-       _refreshToken = refreshToken,
-       _dio = dio ?? Dio(BaseOptions(baseUrl: baseUrl)) {
-    _dio.interceptors.add(
-      ApiInterceptor(
-        tokenStorage: _tokenStorage,
-        dio: _dio,
-        onTokenRefresh: _handleTokenRefresh,
-      ),
+  }) {
+    final effectiveDio = dio ?? Dio(BaseOptions(baseUrl: baseUrl));
+    final interceptor = ApiInterceptor(
+      tokenStorage: tokenStorage,
+      dio: effectiveDio,
+    );
+    effectiveDio.interceptors.add(interceptor);
+    return ApiClient._(
+      dio: effectiveDio,
+      tokenStorage: tokenStorage,
+      interceptor: interceptor,
     );
   }
 
+  ApiClient._({
+    required Dio dio,
+    required TokenStorage tokenStorage,
+    required ApiInterceptor interceptor,
+  }) : _dio = dio,
+       _tokenStorage = tokenStorage,
+       _interceptor = interceptor;
+
   Dio get dio => _dio;
 
-  Future<bool> _handleTokenRefresh() async {
-    final refreshToken = await _tokenStorage.getRefreshToken();
-    if (refreshToken == null || _refreshToken == null) return false;
-    try {
-      final success = await _refreshToken!(refreshToken);
-      return success;
-    } catch (_) {
-      return false;
-    }
+  void setRefreshCallback(Future<bool> Function(String) refresh) {
+    _interceptor.onTokenRefresh = () async {
+      final token = await _tokenStorage.getRefreshToken();
+      if (token == null) return false;
+      return refresh(token);
+    };
   }
 
   Future<Response<T>> get<T>(String path, {Options? options}) async {