User styles form accepts styles over the length limit

[can3p]

SettingsUserStyles.Validate (pkg/forms/form_user_styles.go) calls AddError("styles", ...) for a value over 10000 characters but still returns nil, so the form is saved anyway.

Repro: save the user styles form with 10001 characters of CSS. The error is recorded, but validation passes and the styles are stored.

Expected: validation fails and nothing is saved.

Pinned by the skipped test TestSettingsUserStyles_ValidateTooLongStylesFailsValidation (W2).