Sign-out now forces a full service worker and cache cleanup

Signing out used to occasionally leave you on a stale 404 or briefly show the previous user's dashboard if you signed back in quickly. The root cause was the service worker holding the route table and a few cached responses after the session cookie was cleared.

Sign-out now force-unregisters the service worker, flushes the cache APIs, and clears any precached offline pages — all in the same operation as the cookie clear. The class of "I signed out but the previous state is still showing" bugs is gone.