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/scan.vue

@@ -72,49 +72,33 @@ definePageMeta({
 
 const scannedBarcode = ref<string | null>(null)
 const productData = ref<any>(null)
-const isLookingUp = ref(false)
-const lookupError = ref<string | null>(null)
 const showManualEntry = ref(false)
 
+// Use product lookup composable
+const { lookupProduct, isLoading: isLookingUp, error: lookupError } = useProductLookup()
+
 const handleBarcodeDetected = async (barcode: string) => {
   scannedBarcode.value = barcode
-  lookupError.value = null
-  isLookingUp.value = true
-
-  try {
-    // TODO: Implement product lookup via Edge Function (Issue #24)
-    // For now, create a basic product object
-    await new Promise(resolve => setTimeout(resolve, 1000)) // Simulate API call
-    
-    productData.value = {
-      name: `Product ${barcode}`,
-      brand: 'Unknown Brand',
-      barcode: barcode,
-      image_url: null
-    }
-    
-    lookupError.value = 'Product lookup not yet implemented. Using default data.'
-  } catch (error) {
-    console.error('Product lookup error:', error)
-    lookupError.value = 'Failed to look up product. You can still add it manually.'
-    productData.value = {
-      name: `Product ${barcode}`,
-      barcode: barcode
-    }
-  } finally {
-    isLookingUp.value = false
+  
+  // Fetch product data from Edge Function
+  const data = await lookupProduct(barcode)
+  
+  if (data) {
+    productData.value = data
   }
 }
 
 const addToInventory = () => {
-  // TODO: Implement scan-to-add flow (Issue #25)
-  // Navigate to add form with pre-filled data
+  // Navigate to home page with add form open and pre-filled
   navigateTo({
     path: '/',
     query: {
+      action: 'add',
       barcode: scannedBarcode.value,
-      name: productData.value?.name,
-      brand: productData.value?.brand
+      name: productData.value?.name || undefined,
+      brand: productData.value?.brand || undefined,
+      image_url: productData.value?.image_url || undefined,
+      quantity: productData.value?.quantity || undefined
     }
   })
 }