Missing field-level validation messages

[Sogl]

Summary

grav-plugin-login emits onFormValidationError with only a generic message for several validation failures (e.g. duplicate email/username), but does not provide per-field messages.

As a result, Form plugin templates cannot attach errors to specific inputs (has-errors, inline field messages), and only top-level alert is shown.

Environment

• PHP 8.2
• Latest plugins: login, form

Reproduction

1. Enable public registration with Login plugin.
2. Try to register with an email that already exists.
3. Observe error output in form.

Actual behavior

• Only generic form error is shown (top message).
• No field-level error for email.
• No has-errors class for the email field in default Form rendering.

Expected behavior

• Validation events should include field-scoped messages, for example:

$this->grav->fireEvent('onFormValidationError', new Event([
    'form' => $form,
    'message' => $language->translate(['PLUGIN_LOGIN.EMAIL_NOT_AVAILABLE', $email]),
    'messages' => [
        'email' => [
            $language->translate(['PLUGIN_LOGIN.EMAIL_NOT_AVAILABLE', $email])
        ]
    ]
]));

So Form plugin can render inline error under the correct input.

Where this happens in login.php

Duplicate username/email during registration (processUserRegistration()):
- username check: https://github.com/getgrav/grav-plugin-login/blob/develop/login.php#L827-L835
- email check: https://github.com/getgrav/grav-plugin-login/blob/develop/login.php#L842-L850

Duplicate username/email during profile update (processUserProfile()):
- username check: https://github.com/getgrav/grav-plugin-login/blob/develop/login.php#L999-L1007
- email check: https://github.com/getgrav/grav-plugin-login/blob/develop/login.php#L1017-L1025

These events currently pass message but not messages.

Related issue / same mechanism

grav-plugin-form issue showing the same pattern (generic message without field mapping):

• CaptchaManager doesn't set per-field messages on failure, breaking has-errors highlighting grav-plugin-form#627

That issue is in another plugin path (captcha in Form), but mechanism is identical: without messages[field], inline field error rendering does not work.