Sign in with Google or any OIDC provider

Hive now supports configurable OIDC login. Connect Google as a built-in provider using HIVE_GOOGLE_CLIENT_ID, HIVE_GOOGLE_CLIENT_SECRET, and an optional HIVE_GOOGLE_ALLOWED_DOMAINS allowlist, or connect any generic OIDC provider by pointing HIVE_OIDC_ISSUER at its discovery URL. The login page renders one button per configured provider, and a fix ensures those buttons appear even on public instances so visitors can still sign in when auth is available.