docs: add PWA launch checklist

PWA_CHECKLIST.md

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+# PWA Launch Checklist
+
+## Pre-Production
+
+- [ ] **Replace placeholder icons** with branded designs
+  - Create 192x192px and 512x512px icons
+  - Create maskable variants with safe area padding
+  - Use [Maskable.app](https://maskable.app/) to test maskable icons
+  
+- [ ] **Test install flow**
+  - [ ] Chrome/Edge on desktop
+  - [ ] Chrome on Android
+  - [ ] Safari on iOS (limited PWA support)
+  
+- [ ] **Test offline functionality**
+  - [ ] App shell loads when offline
+  - [ ] Cached images display
+  - [ ] Cached articles accessible
+  - [ ] Local relay still works
+  - [ ] Online/offline toast notifications work
+  
+- [ ] **Run Lighthouse audit**
+  - [ ] PWA score >90
+  - [ ] All installability criteria met
+  - [ ] No console errors
+  
+- [ ] **Test service worker updates**
+  - [ ] Deploy new version
+  - [ ] Verify update notification appears
+  - [ ] Verify refresh loads new version
+
+## Production
+
+- [ ] **Verify manifest serving**
+  - Check `/manifest.webmanifest` is accessible
+  - Verify correct MIME type (`application/manifest+json`)
+  
+- [ ] **Verify service worker serving**
+  - Check `/sw.js` is accessible
+  - Verify correct cache headers (short cache, must-revalidate)
+  
+- [ ] **Verify icons serving**
+  - All icon sizes load correctly
+  - Proper MIME types for PNG files
+  
+- [ ] **Test on real devices**
+  - [ ] iOS Safari (add to home screen)
+  - [ ] Android Chrome (install prompt)
+  - [ ] Desktop Chrome (install button)
+  - [ ] Desktop Edge (install button)
+
+## Post-Launch
+
+- [ ] Monitor service worker registration errors (analytics/logs)
+- [ ] Monitor cache hit rates (optional)
+- [ ] Gather user feedback on install experience
+- [ ] Consider adding advanced PWA features:
+  - [ ] Web Share Target
+  - [ ] Background Sync
+  - [ ] Badging API
+  - [ ] Push Notifications
+
+## Known Limitations
+
+- **iOS Safari**: Limited PWA support, no install prompt (users must "Add to Home Screen" manually)
+- **Firefox**: No install prompt, but PWA features work
+- **Private/Incognito**: Service workers may be disabled
+- **WebSocket**: Not affected by service worker (by design)
+
+## Troubleshooting
+
+### Service Worker Not Registering
+1. Check browser console for errors
+2. Verify `/sw.js` is accessible
+3. Check HTTPS is enabled (required for SW)
+4. Clear browser cache and reload
+
+### Install Prompt Not Showing
+1. Verify manifest is valid (Chrome DevTools > Application > Manifest)
+2. Check all installability criteria (Lighthouse PWA audit)
+3. Try in Chrome/Edge (best PWA support)
+4. Some browsers require user engagement before showing prompt
+
+### Offline Not Working
+1. Check service worker is active (DevTools > Application > Service Workers)
+2. Verify precache manifest was generated (check `dist/sw.js`)
+3. Test after first visit (SW needs initial registration)
+4. Check network tab shows "(from ServiceWorker)" for cached resources
+
+### Icons Not Showing
+1. Verify icon files exist in `public/` directory
+2. Check manifest references correct paths
+3. Verify icon files are valid PNG format
+4. Clear browser cache and reinstall app
+