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/components/inventory/AddItemForm.vue

@@ -131,6 +131,16 @@ const { addInventoryItem, addItemTags } = useInventory()
 const { getUnits } = useUnits()
 const { getTags } = useTags()
 
+const props = defineProps<{
+  initialData?: {
+    barcode?: string
+    name?: string
+    brand?: string
+    image_url?: string
+    quantity?: string
+  }
+}>()
+
 const emit = defineEmits<{
   close: []
   added: [item: any]
@@ -166,6 +176,40 @@ onMounted(async () => {
   if (defaultUnit) {
     form.unit_id = defaultUnit.id
   }
+
+  // Pre-fill from initial data (scan-to-add flow)
+  if (props.initialData) {
+    if (props.initialData.name) {
+      form.name = props.initialData.name
+    }
+    
+    // Add brand to notes if available
+    if (props.initialData.brand) {
+      form.notes = `Brand: ${props.initialData.brand}`
+      
+      if (props.initialData.barcode) {
+        form.notes += `\nBarcode: ${props.initialData.barcode}`
+      }
+    } else if (props.initialData.barcode) {
+      form.notes = `Barcode: ${props.initialData.barcode}`
+    }
+
+    // Parse quantity if available (e.g., "750g")
+    if (props.initialData.quantity) {
+      const quantityMatch = props.initialData.quantity.match(/^([\d.]+)\s*([a-zA-Z]+)$/)
+      if (quantityMatch) {
+        form.quantity = parseFloat(quantityMatch[1])
+        // Try to match unit
+        const unitAbbr = quantityMatch[2].toLowerCase()
+        const matchedUnit = units.value.find(u => 
+          u.abbreviation.toLowerCase() === unitAbbr
+        )
+        if (matchedUnit) {
+          form.unit_id = matchedUnit.id
+        }
+      }
+    }
+  }
 })
 
 // Unit options for select