Sign in with Google or a generic OIDC provider

Hive now supports configurable OIDC sign-in. Operators can set HIVE_OIDC_PROVIDER to google for built-in Google OAuth, complete with domain allowlists and the hd= hint, or to generic for any standards-compliant OIDC issuer. The login page renders the configured provider button, and public instances now correctly show the sign-in buttons so users can authenticate.