修复孩子管理:addChild 返回字符串错误的接收方式

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app/parent/child-manage.tsx

@@ -18,8 +18,8 @@ export function ChildManage({ kids }: { kids: { id: number; name: string }[] })
     const fd = new FormData();
     fd.set("name", name);
     startTransition(async () => {
-      const r = await addChild(fd);
+      const err = await addChild(fd);
-      if (r?.error) setMsg(r.error);
+      if (err) setMsg(err);
       else {
         setName("");
         setMsg("已添加,用孩子设备激活即可");