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/composables/useProductLookup.ts

@@ -0,0 +1,61 @@
+// Composable for product lookup via Edge Function
+
+export interface ProductData {
+  barcode: string
+  name: string
+  brand?: string
+  quantity?: string
+  image_url?: string
+  category?: string
+  cached?: boolean
+}
+
+export const useProductLookup = () => {
+  const supabase = useSupabaseClient()
+  const isLoading = ref(false)
+  const error = ref<string | null>(null)
+
+  const lookupProduct = async (barcode: string): Promise<ProductData | null> => {
+    isLoading.value = true
+    error.value = null
+
+    try {
+      const { data, error: functionError } = await supabase.functions.invoke('product-lookup', {
+        body: { barcode }
+      })
+
+      if (functionError) {
+        console.error('Product lookup error:', functionError)
+        error.value = functionError.message || 'Failed to lookup product'
+        
+        // Return basic product data even on error
+        return {
+          barcode,
+          name: `Product ${barcode}`,
+          cached: false
+        }
+      }
+
+      return data as ProductData
+
+    } catch (err) {
+      console.error('Unexpected error during product lookup:', err)
+      error.value = err instanceof Error ? err.message : 'Unknown error'
+      
+      // Return basic product data even on error
+      return {
+        barcode,
+        name: `Product ${barcode}`,
+        cached: false
+      }
+    } finally {
+      isLoading.value = false
+    }
+  }
+
+  return {
+    lookupProduct,
+    isLoading: readonly(isLoading),
+    error: readonly(error)
+  }
+}