feat(auth): add AdminAuthService.loginWithCredentials + rate_limited AuthFailure code

Convenience wrapper around MarketplacesAuthGateway.loginWithCredentials('admin', ...)
that activates the returned session in one call, for apps building a custom admin
login screen. AuthFailure gains a rate_limited code (HTTP 429, parsed Retry-After)
and an optional status field so 401/403/429 surface distinctly without new plumbing.

No credentials, no client-side comparison, no superadmin concept anywhere in this
package - same code path as any other admin credential login. Inert without a
backend implementing POST {credentialsPath} { login, password, mode: 'admin' }.

Additive, backward-compatible. QR/Yandex flows unmodified.

packages/auth/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@marketplaces/auth",
-  "version": "0.2.0",
+  "version": "0.3.0",
   "description": "Standalone Angular authentication UI and client for marketplaces projects.",
   "module": "dist/fesm2022/marketplaces-auth.mjs",
   "typings": "dist/types/marketplaces-auth.d.ts",