Sign in with Google or any OpenID Connect provider

Hive can now delegate authentication to one or more OIDC providers. Google is supported as a first-class provider via HIVE_GOOGLE_CLIENT_ID/HIVE_GOOGLE_CLIENT_SECRET with an optional HIVE_GOOGLE_ALLOWED_DOMAINS allowlist. Generic providers can be wired up through HIVE_OIDC_ISSUER with client credentials and an optional display name. The login page renders a button for each configured provider, and public instances now show those buttons even when the dashboard itself is open.