feat(api): update category fetch requests to include tree structure

frontend/app/admin/products/AdminProductList.tsx

@@ -80,7 +80,7 @@ export default function AdminProductList() {
   }, [refetchProducts]);
 
   useEffect(() => {
-    fetch('/api/categories')
+    fetch('/api/categories?tree')
       .then((r) => r.json())
       .then((data) => { if (Array.isArray(data)) setCategoryTree(data); })
       .catch(() => {});

frontend/app/admin/products/EditProductForm.tsx

@@ -55,7 +55,7 @@ export default function EditProductForm({ product, onSaved, onDeleted }: Props)
 
   useEffect(() => {
     if (isOpen && categoryTree.length === 0) {
-      fetch('/api/categories')
+      fetch('/api/categories?tree')
         .then((r) => r.json())
         .then((data: unknown) => {
           if (Array.isArray(data)) setCategoryTree(data as CategoryNode[]);