Public or private instance visibility via HIVE_VISIBILITY

Introduced a HIVE_VISIBILITY environment variable (public or private) so instance operators can control whether unauthenticated users may reach the dashboard. In public mode, the dashboard is open to everyone while the login page remains available for admins. In private mode, the plug redirects unauthenticated requests to /login. This replaces the older HIVE_AUTH_MODE setting and decouples the availability of sign-in from the requirement to sign in.