The SW cache name was hardcoded to 'pinchchat-v1', meaning PWA users
would get stale cached assets forever. Now the cache name includes
the package version and build timestamp, so each build creates a new
cache and old ones are cleaned up on activation.
Also adds:
- Periodic SW update check (every 30 min)
- Auto-reload when new SW version is detected
72e9bce4c5
