Error states during onboarding include clear recovery guidance

Users who hit an error during signup or onboarding abandon at much higher rates than users further along in the lifecycle, because they have not yet invested enough to push through friction. Raw error codes, generic "Something went wrong" toasts, or error messages with no recovery action leave the user guessing whether their email is already registered, whether their password was too short, or whether they should retry — and most will not bother. The user-experience taxon treats error-state clarity as a first-session conversion lever, not a polish item.

Map each known error code to a human-readable message with a specific next action. In src/app/(auth)/signup/page.tsx, maintain an errorMessages record keyed by error code, and render the mapped message inline on the offending field rather than in a generic toast.

const errorMessages: Record<string, string> = {
  'email-exists': 'This email is already registered — try signing in instead.',
  'invalid-password': 'Password must be at least 8 characters.',
  'rate-limited': 'Too many attempts. Please wait a moment.',
};