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/AGENTS.md

@@ -45,3 +45,42 @@ For important screens, add widget tests that reduce layout-regression risk:
 - Do not skip design container layers.
 - Do not start implementation before retrieving design variables.
 - Do not hardcode colors; use design variables.
+
+## UI Feedback System
+
+**MUST use the Toast system for all user feedback messages.**
+
+### Components
+
+| Component | Use Case | Example |
+|-----------|----------|---------|
+| `Toast.show()` | Global temporary notifications | Success/error feedback after action |
+| `AppBanner` | Inline form validation errors | Login form error message |
+
+### Toast Types
+
+```dart
+enum ToastType { info, success, warning, error }
+```
+
+### Usage Examples
+
+**Global Toast (auto-dismiss):**
+```dart
+Toast.show(context, '保存成功', type: ToastType.success);
+Toast.show(context, '网络错误', type: ToastType.error);
+Toast.show(context, '正在加载...', type: ToastType.info, duration: Duration(seconds: 3));
+```
+
+**Inline Banner (persistent):**
+```dart
+AppBanner(message: '邮箱或密码错误', type: ToastType.error)
+AppBanner(message: '请检查输入', type: ToastType.warning)
+```
+
+### Rules
+
+- Use `Toast` for transient feedback that auto-dismisses
+- Use `AppBanner` for persistent inline messages (form errors)
+- Do NOT create custom SnackBar, Dialog, or Banner components
+- Do NOT use raw `ScaffoldMessenger`