Sign in with Google or a custom OIDC provider

Hive now supports signing in through Google or a generic OpenID Connect provider. The login page renders one button per configured provider, and the auth flow routes through /auth/:provider. For Google, set HIVE_OIDC_PROVIDER=google plus HIVE_OIDC_CLIENT_ID and HIVE_OIDC_CLIENT_SECRET. For a custom IdP, set HIVE_OIDC_PROVIDER=generic and supply the issuer, authorize, token, and userinfo URLs. A bug that hid provider buttons on public instances was also fixed, so the login page now correctly shows configured providers even when the dashboard is public.