feat: PWA manifest and service worker for installability and offline cache

- Add manifest.json with app name, icons, standalone display mode
- Add sw.js service worker with stale-while-revalidate caching for
  static assets and automatic old-cache cleanup on version bump
- Register service worker on init; link manifest in HTML head
- Chrome/Edge will show "Install" prompt; app launches in standalone window

Author: SysAdminDoc