Handle Firebase client-side token and access to protected pages - firebase

Question

I'm using Firebase auth to login with Facebook, Google and email/pass. Basically, everything runs client-side, I make a call to Firebase and I receive an object containing an access token (that is a JWT ID Token), a customer id and its email. When I get this object, I put it into a persistent store (local storage, I know it's bad) and I perform an API call to one of my sveltekit endpoint that will in turn make another API call to a backend API (written in Go) to get all the user informations: firstname, lastname, phone, address, stats etc. To give a little bit of context, below is a diagram to illustrate what's happening when a user sign-in using Facebook.
Up to now, I just put the Firebase object into a store and just check if the information are there to allow access to a particular page. This check is done in the +layout.svelte page of the directory containing the page to be protected. It looks like something like this:
onMount(() => {
// redirect if not authenticated
if (browser && !$authStore?.uid) goto(`/auth/sign-in`);
});
It's probably not a good thing especially since my store persists in the local storage and therefore is prone to some javascript manipulation.
From my understanding, there's at least 2 things that could be better implemented but I may be wrong:
Set the access token in an httponly cookie straight after receiving it from Firebase. This would avoid storing the access token (that is a JWT) in the local storage and It could be used to authorize access or not to some protected pages...
I receive the Firebase authentication object on client-side buthttp only cookie can only be created from server side. I thought about sending the object as a payload to a POST sveltekit endpoint (like /routes/api/auth/token/+server.js) and set the cookie from here but apparently cookies is not available in a sveltekit endpoint.
So, how and where should I set this cookie ? I see that cookies is available in the load function of a +layout.server.js file, as well as in the handle function of a hooks.server.js file, but I don't see the logic here.
Populate locals.userwith the authenticated user once I've performed a call to my backend. Well, here, it's not obvious to me because I think point 1) would be enough to manage access to protected pages, but I see that a check of locals.user is something I've seen elsewhere.
I tried to set locals.user in the sveltekit endpoint that is making the API call to the backend API:
// /routes/api/users/[uid]/+server.js
import { json } from "#sveltejs/kit";
import axios from "axios";
import { GO_API_GATEWAY_BASE_URL } from "$env/static/private";
export async function GET({ request, locals, params }) {
try {
const config = {
method: "get",
baseURL: GO_API_GATEWAY_BASE_URL,
url: `/users/${params.uid}`,
headers: {
Authorization: `Bearer ${uidToken}`, // <-- the Firebase ID Token
},
withCredentials: true,
};
const res = await axios(config);
// set locals
locals.user = json(res.data); // <--- DOESN'T SEEM TO WORK
return json(res.data);
} catch (error) {...}
}
...but in the +layout.server.js page that I've created I see nothing:
// routes/app/protected_pages/+layout.server.js
import { redirect } from "#sveltejs/kit";
export function load({ locals }) {
console.log(locals); // <----- contains an empty object: {}
if (!locals.user) throw redirect(302, "/auth/sign-in");
}
Thank you so much for your help