herdr web ui v0.2.1

Fixed

• Updates now replace the update supervisor too. server/managed.ts became a small launcher that
  runs the active release's supervisor; after an install passes its health check the supervisor
  hands over to the new one (one more brief reconnect), and a new supervisor that cannot start is
  replaced by the previous one, with the failure shown in Settings → Updates.
• Release CI skips the one updater test that needs a live herdr.