Google OIDC is now a built-in provider preset

Hive now supports a single OIDC provider per instance, selected with HIVE_OIDC_PROVIDER=google or generic. Set it to google and supply HIVE_OIDC_CLIENT_ID and HIVE_OIDC_CLIENT_SECRET to use Google’s built-in endpoints without needing to paste URLs. Generic providers still work with HIVE_OIDC_AUTHORIZE_URL, HIVE_OIDC_TOKEN_URL, and the other standard OIDC variables. The login page renders one button for the configured provider, and the callback path stays /auth/google/callback or /auth/oidc/callback.