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/pages/offline.vue

@@ -0,0 +1,69 @@
+<template>
+  <div class="flex flex-col items-center justify-center min-h-screen p-8 text-center">
+    <UIcon name="i-heroicons-wifi-slash" class="w-24 h-24 text-gray-400 mb-6" />
+    
+    <h1 class="text-3xl font-bold text-gray-900 mb-4">
+      You're Offline
+    </h1>
+    
+    <p class="text-gray-600 mb-8 max-w-md">
+      No internet connection detected. Some features may be limited, but you can still:
+    </p>
+    
+    <div class="space-y-3 mb-8 text-left max-w-md">
+      <div class="flex items-start gap-3">
+        <UIcon name="i-heroicons-check-circle" class="w-6 h-6 text-emerald-500 mt-0.5" />
+        <span class="text-gray-700">View cached inventory items</span>
+      </div>
+      <div class="flex items-start gap-3">
+        <UIcon name="i-heroicons-check-circle" class="w-6 h-6 text-emerald-500 mt-0.5" />
+        <span class="text-gray-700">Scan barcodes (will sync when online)</span>
+      </div>
+      <div class="flex items-start gap-3">
+        <UIcon name="i-heroicons-check-circle" class="w-6 h-6 text-emerald-500 mt-0.5" />
+        <span class="text-gray-700">Browse previously loaded data</span>
+      </div>
+    </div>
+    
+    <UButton
+      color="emerald"
+      size="lg"
+      @click="retry"
+      :loading="retrying"
+    >
+      <template #leading>
+        <UIcon name="i-heroicons-arrow-path" />
+      </template>
+      Try Again
+    </UButton>
+  </div>
+</template>
+
+<script setup lang="ts">
+const retrying = ref(false)
+
+async function retry() {
+  retrying.value = true
+  
+  try {
+    // Test if we're back online
+    const response = await fetch('/api/health', { method: 'HEAD' })
+    if (response.ok) {
+      // We're online! Go back
+      window.location.reload()
+    }
+  } catch (error) {
+    // Still offline
+    setTimeout(() => {
+      retrying.value = false
+    }, 1000)
+  }
+}
+
+// Auto-retry when online event fires
+if (typeof window !== 'undefined') {
+  window.addEventListener('online', () => {
+    window.location.reload()
+  })
+}
+</script>