Add PWA and mark delivery receipts done in Phase 2 roadmap

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

DESIGN.md

@@ -445,6 +445,15 @@ warzone.wasm      # browser client (via wasm-pack)
 - [x] TUI client (ratatui)
 - [x] Web client (WASM)
 - [x] WebSocket real-time push
+- [x] Delivery receipts (sent/delivered/read)
+- [ ] Progressive Web App (PWA)
+  - Web manifest with standalone display mode
+  - Service worker for offline shell + notification support
+  - Install prompt (Android Chrome "Add to Home Screen")
+  - iOS: apple-mobile-web-app-capable meta tags
+  - Push notifications via service worker (when tab unfocused)
+  - Offline: show cached identity + "reconnecting" state
+  - App icon (SVG, maskable)
 - [ ] Encrypted local message history & cloud backup
   - Messages encrypted at rest using key derived from seed (HKDF, info="warzone-history")
   - No extra password needed — if you have your seed, you can read your history