Bug: Password validation error when using Cyrillic characters during registration

### Summary

When registering a new user on the /signup page, the system fails to validate Cyrillic characters in the password field, allowing invalid input or causing an unhandled error.

### Reproduction steps

```bash
1. Navigate to the registration page /signup.
2. Enter Nickname: tester123 and Email: test@example.com.
3. Enter 12345678ы into the Password field.
4. Re-enter 12345678ы into the Password Confirmation field.
5. Click the Submit button.
```

### Expected result

The system displays a validation error "Should contain at least one letter" (or blocks Cyrillic input) and prevents form submission.

### Actual result

The form is submitted without validating Cyrillic characters (the user is registered with Cyrillic characters in the password), or a server error occurs without displaying a validation error message.

### Browsers

Chrome

### OS

Windows

首先在 Windows 上的 Chrome 中，使用密码 12345678ы 在 /signup 页面复现该问题，然后跟踪注册验证和提交处理流程。完成的标准是：西里尔字母输入得到一致处理，显示预期的验证错误或阻止该输入，并且注册不会继续，也不会因未处理的服务器错误而失败。

由索引模型根据 Issue 内容生成。