* feat: add Web Push API support and PWA integration
Add web Push API with subscribe/unsubscribe and visibility endpoints
Introduce usePushVisibilityBeacon and useSessionDeepLink hooks
Integrate PWA with service worker, registerSW, and VAPID key persistence
* feat: add heartbeat visibility beacon for web runtime
Add a 10s heartbeat to ping visibility while visible
Subscribe to visibilitychange, focus, blur, pageshow, and pagehide events to report state
Clear heartbeat interval on unmount to avoid leaks
