Add Progressive Web App functionality

- Add web app manifest for home screen installation
- Implement service worker with offline caching strategy
- Create offline fallback page with auto-reconnect
- Generate PWA icons in multiple sizes (72px-512px)
- Add PWA meta tags and Apple Touch icons to all pages
- Register service worker with graceful degradation
- Update documentation with PWA installation instructions
- Add browserconfig.xml for Windows tile support

Features:
- Installable on mobile and desktop
- Offline functionality with cached resources
- App-like experience in standalone mode
- Automatic updates when online
- Works seamlessly with existing progressive enhancement

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

public/browserconfig.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<browserconfig>
+    <msapplication>
+        <tile>
+            <square150x150logo src="/icons/icon-152.svg"/>
+            <square310x310logo src="/icons/icon-384.svg"/>
+            <TileColor>#2196F3</TileColor>
+        </tile>
+    </msapplication>
+</browserconfig>