- Add `AdminAuthService.loginWithCredentials(credentials)`, a convenience wrapper around the existing `MarketplacesAuthGateway.loginWithCredentials('admin', credentials)` call that activates the returned session on success. Lets a consuming app build a fully custom admin login screen with one call instead of wiring the gateway token directly.
`AuthFailure` gained a `rate_limited` code (mapped from HTTP 429, with a parsed `retryAfterSeconds` from the `Retry-After` header) plus an optional `status` field, so a custom login UI can distinguish invalid credentials, rate-limiting/lockout, and other backend failures without new plumbing. Existing `invalid_credentials`/`backend` mapping is unchanged.
This ships **no credentials, no username/password comparison, and no notion of a privileged account** anywhere in this package — it is the exact same code path as any other admin credential login. It is inert until a real backend implements `POST {credentialsPath}` with `{ login, password, mode: 'admin' }` and returns a verified, audit-logged session for whatever account the backend chooses to treat specially. Additive and backward-compatible with the existing `qr`/`credentials`/`yandex` flows, which are unmodified.