feat: add PWA + Capacitor mobile app for iOS/Android stores

[Wool-xing]

Summary

Mobile app ready for App Store and Google Play publishing:

PWA (works NOW in mobile browsers):

• manifest.json — installable PWA with icons + theme
• service worker — offline shell caching
• Responsive CSS — phone notch safe area, small-screen nav
• "Add to Home Screen" support on iOS Safari / Android Chrome

Capacitor (for native stores):

• iOS: npx cap open ios → Xcode → Archive → App Store
• Android: npx cap open android → Android Studio → Build → Play Store
• Config in mobile/capacitor.config.json

Compatibility (mobile/COMPATIBILITY.md):

• Windows 10/11, macOS 12-15, iOS 14+, Android 8+
• All major browsers (Chrome/Firefox/Safari/Edge 90+)

How to publish

cd runtime/web && npm run build   # build web UI
cd mobile && npm install           # install Capacitor
npx cap add ios                    # add iOS platform
npx cap open ios                   # open in Xcode → publish