- Backend: explicitly return 409 EMAIL_TAKEN when the email already
exists, instead of Better Auth's silent 200 (common after migration).
- Frontend: split error handling — network failure, non-JSON body
(502/proxy), and meaningful server errors get distinct messages; the
"email already registered" case shows login / reset-password links
instead of a generic "connection error".
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>