fix mistake

frontend/src/components/Header.js

@@ -19,7 +19,7 @@ export default function Header({ selection, setSelection }) {
         .get(`${process.env.REACT_APP_BASE_URL_BACK}/auth`, { withCredentials: true })
         .then((res) => {
           setUser(res.data);
-          localStorage.setItem("user", res.data);
+          localStorage.setItem("user", JSON.stringify(res.data));
         })
         .catch((e) => console.log(e));
     }