feat: configure service worker and offline support (#34)

- Enhance Workbox configuration with comprehensive caching strategies
- Add separate caching for Supabase REST API, Storage, and Auth
- Configure Open Food Facts API caching (30-day cache)
- Add offline fallback page with retry functionality
- Create useOnlineStatus composable for network monitoring
- Add OfflineBanner component for user feedback
- Configure skipWaiting and clientsClaim for instant updates
- Cache Google Fonts and product images

Caching strategies:
- Network-first: Supabase REST API (fresh data priority)
- Network-only: Auth endpoints (never cache sensitive auth)
- Cache-first: Images, fonts, product data (performance)
- Offline fallback: /offline page for failed navigations

Closes #34

app/composables/useOnlineStatus.ts

@@ -0,0 +1,47 @@
+/**
+ * Composable to track online/offline status
+ * 
+ * Usage:
+ * const { isOnline, wasOffline } = useOnlineStatus()
+ * 
+ * watch(isOnline, (online) => {
+ *   if (online && wasOffline.value) {
+ *     // User came back online, sync data
+ *   }
+ * })
+ */
+export function useOnlineStatus() {
+  const isOnline = ref(true)
+  const wasOffline = ref(false)
+
+  if (process.client) {
+    // Initial state
+    isOnline.value = navigator.onLine
+
+    // Listen for online/offline events
+    const updateOnlineStatus = () => {
+      const online = navigator.onLine
+      
+      if (!online && isOnline.value) {
+        // Just went offline
+        wasOffline.value = true
+      }
+      
+      isOnline.value = online
+    }
+
+    window.addEventListener('online', updateOnlineStatus)
+    window.addEventListener('offline', updateOnlineStatus)
+
+    // Cleanup on unmount
+    onUnmounted(() => {
+      window.removeEventListener('online', updateOnlineStatus)
+      window.removeEventListener('offline', updateOnlineStatus)
+    })
+  }
+
+  return {
+    isOnline: readonly(isOnline),
+    wasOffline: readonly(wasOffline)
+  }
+}