- Add web app manifest, maskable/apple-touch icons, and a production-only
service worker that caches the app shell for offline start
- Improve mobile UX: responsive breakpoints from ~320px, 44px touch
targets, safe-area insets, dvh heights, 16px inputs to avoid iOS zoom
- Serve .webmanifest with correct content type in the Go static handler
- Document PWA behavior and mobile features in the README
