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/shared/widgets/app_button.dart

@@ -4,7 +4,6 @@ import '../../core/theme/design_tokens.dart';
 class AppButton extends StatelessWidget {
   final String text;
   final VoidCallback? onPressed;
-  final bool isPrimary;
   final bool isOutlined;
   final double height;
   final bool isLoading;
@@ -13,7 +12,6 @@ class AppButton extends StatelessWidget {
     super.key,
     required this.text,
     this.onPressed,
-    this.isPrimary = true,
     this.isOutlined = false,
     this.height = 44,
     this.isLoading = false,