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/sw.js

@@ -0,0 +1,143 @@
+// Service Worker for Great Lakes Ice Report PWA
+const CACHE_NAME = 'ice-report-v1';
+const OFFLINE_URL = '/offline.html';
+
+// Files to cache for offline functionality
+const CACHE_FILES = [
+  '/',
+  '/index.html',
+  '/admin.html',
+  '/privacy.html',
+  '/style.css',
+  '/app.js',
+  '/admin.js',
+  '/utils.js',
+  '/manifest.json',
+  OFFLINE_URL
+];
+
+// Install event - cache essential files
+self.addEventListener('install', (event) => {
+  console.log('Service Worker: Installing...');
+  
+  event.waitUntil(
+    caches.open(CACHE_NAME)
+      .then((cache) => {
+        console.log('Service Worker: Caching files');
+        return cache.addAll(CACHE_FILES);
+      })
+      .then(() => {
+        console.log('Service Worker: Files cached successfully');
+        return self.skipWaiting();
+      })
+      .catch((error) => {
+        console.error('Service Worker: Cache failed:', error);
+      })
+  );
+});
+
+// Activate event - clean up old caches
+self.addEventListener('activate', (event) => {
+  console.log('Service Worker: Activating...');
+  
+  event.waitUntil(
+    caches.keys()
+      .then((cacheNames) => {
+        return Promise.all(
+          cacheNames
+            .filter((cacheName) => cacheName !== CACHE_NAME)
+            .map((cacheName) => {
+              console.log('Service Worker: Deleting old cache:', cacheName);
+              return caches.delete(cacheName);
+            })
+        );
+      })
+      .then(() => {
+        console.log('Service Worker: Activated');
+        return self.clients.claim();
+      })
+  );
+});
+
+// Fetch event - serve cached content when offline
+self.addEventListener('fetch', (event) => {
+  // Only handle GET requests
+  if (event.request.method !== 'GET') {
+    return;
+  }
+
+  // Skip cross-origin requests
+  if (!event.request.url.startsWith(self.location.origin)) {
+    return;
+  }
+
+  event.respondWith(
+    caches.match(event.request)
+      .then((response) => {
+        // Return cached version if available
+        if (response) {
+          return response;
+        }
+
+        // Try to fetch from network
+        return fetch(event.request)
+          .then((response) => {
+            // Don't cache non-successful responses
+            if (!response || response.status !== 200 || response.type !== 'basic') {
+              return response;
+            }
+
+            // Clone the response for caching
+            const responseToCache = response.clone();
+
+            // Cache successful responses for static assets
+            if (event.request.url.match(/\.(js|css|html|png|jpg|jpeg|gif|svg|ico|woff|woff2)$/)) {
+              caches.open(CACHE_NAME)
+                .then((cache) => {
+                  cache.put(event.request, responseToCache);
+                });
+            }
+
+            return response;
+          })
+          .catch(() => {
+            // If network fails and we're requesting an HTML page, show offline page
+            if (event.request.headers.get('accept').includes('text/html')) {
+              return caches.match(OFFLINE_URL);
+            }
+            
+            // For other requests, just fail
+            throw new Error('Network failed and no cache available');
+          });
+      })
+  );
+});
+
+// Background sync for when connection is restored
+self.addEventListener('sync', (event) => {
+  if (event.tag === 'background-sync') {
+    console.log('Service Worker: Background sync triggered');
+    // Could implement queued location submissions here
+  }
+});
+
+// Push notifications (for future use)
+self.addEventListener('push', (event) => {
+  if (event.data) {
+    const data = event.data.json();
+    console.log('Service Worker: Push message received:', data);
+    
+    // Could show notifications about severe weather warnings
+    const options = {
+      body: data.body,
+      icon: '/icons/icon-192.png',
+      badge: '/icons/icon-72.png',
+      tag: 'ice-report',
+      renotify: true
+    };
+    
+    event.waitUntil(
+      self.registration.showNotification(data.title, options)
+    );
+  }
+});