server : remove the built-in UI's service worker when the UI is not served (#29565)

With --path or --no-ui, /sw.js returned 404, and a 404 does not remove a service worker, so browsers kept showing the cached built-in UI. Serve a worker that unregisters itself, clears its caches and reloads open tabs. A sw.js in the --path folder is still served first.

Assisted-by: Claude Opus 5.5