Wires the existing @marketplaces/auth credentials gateway into the admin
Telegram login dialog behind a toggle. Targets the documented but not-yet-
live POST /api/identity/v1/session contract (BACKEND-INTEGRATION.md §1.2);
any failure - 404 today, a real auth error once the backend ships - shows
one generic "try another way to sign in" message, never the raw error.
Also adds public/favicon.ico (was 404ing; reuses existing app icon).
