PWA enabled:
- /manifest.json and /sw.js are served at root scope for installability.
- Install buttons appear when browser fires beforeinstallprompt.
- Works on HTTPS (required) or localhost.
- Offline fallback route: /offline/

To test install prompt:
1) Run on https domain or localhost.
2) Open in Chrome, check Application tab -> Manifest.
3) Use 'Install' in address bar or click Install button.