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

Start by reproducing the issue on the /signup page with the password 12345678ы in Chrome on Windows, then trace the registration validation and submission handling. Done means Cyrillic input is handled consistently, the expected validation error is shown or the input is blocked, and registration does not proceed or fail with an unhandled server error.

索引モデルが issue の本文から書いたものです。