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/form_inputs/form_inputs.dart

@@ -0,0 +1,52 @@
+import 'package:formz/formz.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 '请输入用户名';
+    if (value.length < 3) return '用户名至少 3 个字符';
+    if (value.length > 30) return '用户名最多 30 个字符';
+    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+$');
+
+  @override
+  String? validator(String value) {
+    if (value.isEmpty) return '请输入邮箱';
+    if (!_regex.hasMatch(value)) return '邮箱格式不正确';
+    return null;
+  }
+}
+
+class Password extends FormzInput<String, String> {
+  const Password.pure() : super.pure('');
+  const Password.dirty([super.value = '']) : super.dirty();
+
+  @override
+  String? validator(String value) {
+    if (value.isEmpty) return '请输入密码';
+    if (value.length < 6) return '密码至少 6 个字符';
+    return null;
+  }
+}
+
+class VerificationCode extends FormzInput<String, String> {
+  const VerificationCode.pure() : super.pure('');
+  const VerificationCode.dirty([super.value = '']) : super.dirty();
+
+  @override
+  String? validator(String value) {
+    if (value.isEmpty) return '请输入验证码';
+    if (!RegExp(r'^\d{6}$').hasMatch(value)) return '验证码必须是 6 位数字';
+    return null;
+  }
+}