express-vue | A simple , Node starter project | Runtime Evironment library

What Im trying to achieve: I want to create a computed property that depends on the BOM screen (https://www.w3schools.com/js/js_window_screen.asp). somthing like:

Im using express-vue (https://github.com/express-vue/express-vue)

(A package that should do the ssr for me).

Now I wanted to integrate vuex into my project and I'm little confused on how to do that. I can use the cdn link, but I dont know where I should put this code:

Ok, so I'm just starting to learn Vue.js, and man, it's so hard to do things that are very simple when just using EJS for example. I'm close to abandon Vue for my current project, since I just don't know how to pass res.locals.something from Express server to Vue frontend. By the way, it's Passport.js thing - when authenticated, user should be redirected, but I have to pass the info whether user has logged in or not to Vue (res.locals.isLogged = req.isAuthenticated();), and that seems impossible with my current (close to 0) Vue.js skills... The only solution I found was using ajax (axios was my choice) request on the client side, targeting /login/facebook route on the server, and then I could pass the response from Express to Vue, but it cannot work because of the damned CORS issue. So, I cannot use ajax to retrieve the data from Express, and Express and Vue are not natively connected like Express and EJS or Pug for example.

In short - does anyone know of a simple way to pass Express variable to Vue, not including Vue SSR, Express-vue module etc.?

P.S. I'm not using Webpack or anything similar (so, no .vue files etc.) - just a simple index.html file with Vue loaded from CDN.