Six tests covering: load() exposes ?registered and ?reason; action returns 400
on missing email; 401 with INVALID_CREDENTIALS on backend reject; success
re-emits fa_session and deletes legacy auth_token; 500 when backend omits
fa_session in Set-Cookie. Closes the frontend coverage gap on the credential-
handling logic that moved out of the Java side. Addresses PR #612 / Sara S1.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>