Previously "Continue" on the token step only checked the field was
non-empty; a wrong token wasn't caught until the final submit, after the
user had filled in email + password. Add a non-burning verify:
- backend: POST /setup/verify-token constant-time compares the token
without consuming it (createInitialAdmin still claims it atomically on
submit), gated on no-admin-exists and rate-limited like /setup/admin.
- frontend: step-1 "Continue" calls verifyToken and only advances on a
valid token; a wrong token shows the invalidToken error on the field,
429 -> too-many-attempts, 409 -> redirect to login.
Adds integration tests for accept-without-burn / reject / closed-once-set.
Luca
b0912c7427