Sign in with Google or a generic OIDC provider

Hive now supports signing in through Google, a generic OpenID Connect provider, or both at the same time. When you configure the environment variables, the login page shows one button per enabled provider, and the callback routes are handled under /auth/:provider. Google is preset with hardcoded endpoints, while the generic provider can point to any issuer with a .well-known/openid-configuration endpoint. Operators can also restrict sign-in to specific email domains via HIVE_GOOGLE_ALLOWED_DOMAINS and HIVE_OIDC_ALLOWED_DOMAINS.