Private or public dashboard visibility via HIVE_VISIBILITY

Hive can now require users to sign in before they can access the dashboard. Set the HIVE_VISIBILITY environment variable to private to gate routes behind authentication, or leave it as public to let anyone browse. Login itself remains available either way so admins can sign in to a public instance. The setting is enforced by a new RequireAuthenticated plug that redirects unauthenticated visitors to /login.