feat: implement scan-to-add flow (#25)

- Create useProductLookup composable
- Integrate real product lookup in scan page
- Add query parameter handling in index.vue
- Pre-fill AddItemForm from scan data
- Parse quantity and unit from product data
- Include barcode and brand in notes

Complete end-to-end scan workflow:
1. Scan barcode
2. Fetch from Open Food Facts
3. Navigate to inventory with data
4. Pre-filled add form
5. One-click add to inventory

Closes #25

app/pages/index.vue

@@ -28,7 +28,8 @@
     <div v-if="showAddForm" class="fixed inset-0 z-50 flex items-start justify-center pt-20 px-4 bg-black/50">
       <div class="w-full max-w-lg">
         <AddItemForm 
-          @close="showAddForm = false"
+          :initial-data="prefilledData"
+          @close="handleCloseAddForm"
           @added="handleItemAdded"
         />
       </div>
@@ -56,13 +57,42 @@ definePageMeta({
   layout: 'default'
 })
 
+const route = useRoute()
+const router = useRouter()
+
 const showAddForm = ref(false)
 const editingItem = ref<any>(null)
 const refreshKey = ref(0)
 const inventoryListRef = ref()
+const prefilledData = ref<any>(null)
+
+// Handle scan-to-add flow (Issue #25)
+onMounted(() => {
+  if (route.query.action === 'add') {
+    // Pre-fill data from query params (from scan)
+    prefilledData.value = {
+      barcode: route.query.barcode as string || undefined,
+      name: route.query.name as string || undefined,
+      brand: route.query.brand as string || undefined,
+      image_url: route.query.image_url as string || undefined,
+      quantity: route.query.quantity as string || undefined,
+    }
+    
+    showAddForm.value = true
+    
+    // Clean up URL
+    router.replace({ query: {} })
+  }
+})
+
+const handleCloseAddForm = () => {
+  showAddForm.value = false
+  prefilledData.value = null
+}
 
 const handleItemAdded = (item: any) => {
   showAddForm.value = false
+  prefilledData.value = null
   // Reload the inventory list
   inventoryListRef.value?.reload()
 }