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/manifest.json

@@ -0,0 +1,75 @@
+{
+  "name": "Great Lakes Ice Report",
+  "short_name": "Ice Report",
+  "description": "Community-driven winter road conditions and icy hazards tracker for the Great Lakes region",
+  "start_url": "/",
+  "display": "standalone",
+  "orientation": "portrait-primary",
+  "background_color": "#ffffff",
+  "theme_color": "#2196F3",
+  "lang": "en-US",
+  "scope": "/",
+  "categories": ["weather", "transportation", "utilities"],
+  "icons": [
+    {
+      "src": "/icons/icon-72.svg",
+      "sizes": "72x72",
+      "type": "image/svg+xml",
+      "purpose": "any"
+    },
+    {
+      "src": "/icons/icon-96.svg",
+      "sizes": "96x96",
+      "type": "image/svg+xml",
+      "purpose": "any"
+    },
+    {
+      "src": "/icons/icon-128.svg",
+      "sizes": "128x128",
+      "type": "image/svg+xml",
+      "purpose": "any"
+    },
+    {
+      "src": "/icons/icon-144.svg",
+      "sizes": "144x144",
+      "type": "image/svg+xml",
+      "purpose": "any"
+    },
+    {
+      "src": "/icons/icon-152.svg",
+      "sizes": "152x152",
+      "type": "image/svg+xml",
+      "purpose": "any"
+    },
+    {
+      "src": "/icons/icon-192.svg",
+      "sizes": "192x192",
+      "type": "image/svg+xml",
+      "purpose": "any"
+    },
+    {
+      "src": "/icons/icon-384.svg",
+      "sizes": "384x384",
+      "type": "image/svg+xml",
+      "purpose": "any"
+    },
+    {
+      "src": "/icons/icon-512.svg",
+      "sizes": "512x512",
+      "type": "image/svg+xml",
+      "purpose": "any"
+    },
+    {
+      "src": "/icons/icon-192-maskable.svg",
+      "sizes": "192x192",
+      "type": "image/svg+xml",
+      "purpose": "maskable"
+    },
+    {
+      "src": "/icons/icon-512-maskable.svg",
+      "sizes": "512x512",
+      "type": "image/svg+xml",
+      "purpose": "maskable"
+    }
+  ]
+}