Choose public or private dashboard access with HIVE_VISIBILITY

A new HIVE_VISIBILITY environment variable lets operators choose whether the dashboard is public (default) or private. Public instances allow anyone to browse without signing in, while private instances redirect unauthenticated visitors to /login. Login remains available in both modes so admins can sign in to public instances, and the login page now correctly shows configured provider buttons and a “Continue without signing in” option on public instances.