Configurable OIDC login with Google and other providers

Hive 0.2.0 ships a configurable OIDC login experience. Operators can wire up multiple OIDC providers and get Google as a built-in option alongside a generic OpenID Connect provider, and the login shell itself is configurable from config/runtime.exs via the HIVE_OIDC_* env vars. The login page now shows the configured provider buttons even on public instances, and the page was redesigned to match the Tuist login style so users land on a familiar screen when signing in. Under the hood the hand-rolled OAuth flow was replaced by Ueberauth + ueberauth_oidcc.