Function
The worker exists for one reason, and it is not offline support: Chromium
will not fire beforeinstallprompt for a site whose service worker has no
fetch handler, so without one the install prompt can never be offered. The
worker itself caches nothing — see public/sw.js.
Nothing waits for it. The registration is started and the promise is dropped, because no part of the app behaves differently before or after it resolves.
A browser with no service worker support, or one that refuses the registration — a private window, a blocked origin, a user who turned it off — leaves the app exactly as it is without one: the install prompt is never offered, and everything else works. That is a supported state, not an error, which is why the rejection is swallowed rather than surfaced.
Client hook: registers the app's service worker, once, after mount.