refactor(actions): return only serializable fields in product actions

frontend/app/kvitto/actions.ts

@@ -20,7 +20,13 @@ export async function createProductAction(name: string) {
     throw new Error(e.message ?? `HTTP ${res.status}`);
   }
 
-  return res.json();
+  const product = await res.json();
+  // Return only serializable fields (Dates can't be serialized across RSC boundary)
+  return {
+    id: product.id,
+    name: product.name,
+    canonicalName: product.canonicalName,
+  };
 }
 
 export async function updateProductCategoryAction(productId: number, categoryId: number) {
@@ -37,5 +43,12 @@ export async function updateProductCategoryAction(productId: number, categoryId:
     throw new Error(e.message ?? `HTTP ${res.status}`);
   }
 
-  return res.json();
+  const product = await res.json();
+  // Return only serializable fields
+  return {
+    id: product.id,
+    name: product.name,
+    canonicalName: product.canonicalName,
+    categoryId: product.categoryId,
+  };
 }