Service worker fully flushes on sign-out

One more layer of the sign-out cleanup: when you sign out, the service worker now force-unregisters, the cache APIs flush, and any precached offline pages get invalidated alongside the session cookie.

This is the change that finally kills the class of bugs where users would sign out, sign back in as a different user on the same browser, and briefly see a flash of the previous user's data because the service worker was still serving cached responses.

If you share a browser with someone (designer + dev pairing, a demo laptop, a customer-success terminal), this is the change that lets account switching feel safe.