Sign in with Google or a generic OIDC provider

Hive now supports configurable OIDC login through either Google or a generic OIDC provider. Use HIVE_OIDC_PROVIDER=google or generic, then set the corresponding client ID, client secret, and allowed domains. For Google, Hive supplies the endpoints automatically and adds a hosted-domain hint when a single allowed domain is configured. For generic OIDC, operators provide the discovery URL, authorization endpoint, token endpoint, and userinfo endpoint.