refactor(categories): change controller route from 'api/categories' to 'categories'

fix(products): update category fetch logic to ensure data is an array
refactor(products): simplify GET handler by integrating authentication directly

frontend/app/admin/products/EditProductForm.tsx

@@ -43,7 +43,9 @@ export default function EditProductForm({ product }: Props) {
     if (isOpen && categoryTree.length === 0) {
       fetch('/api/categories')
         .then((r) => r.json())
-        .then((data: CategoryNode[]) => setCategoryTree(data))
+        .then((data: unknown) => {
+          if (Array.isArray(data)) setCategoryTree(data as CategoryNode[]);
+        })
         .catch(() => {});
     }
   }, [isOpen]);