Introduce LegacySignupErrorEvent to map backend failures from
POST /auth/signup: 400 → emailAlreadyExists, 403 → invalidField,
429 → tooManyAttempts, timeout → network. The view state now
separates validationErrorKey (pre-submit i18n keys) from apiErrorEvent
(typed API outcomes), and the screen listens to both to show proper
i18n messages instead of leaking raw backend text.