Sign in with Google or any OIDC provider

Hive now ships with Google as a built-in OIDC provider, alongside a generic OIDC option. Configure Google via HIVE_GOOGLE_CLIENT_ID and HIVE_GOOGLE_CLIENT_SECRET, and optionally restrict who can sign in with HIVE_GOOGLE_ALLOWED_DOMAINS (a single entry also pre-selects that Google-hosted domain on the consent screen). Any other identity provider can be wired up via HIVE_OIDC_ISSUER, HIVE_OIDC_CLIENT_ID, and HIVE_OIDC_CLIENT_SECRET. The Login page renders one button per configured provider.