1. If you are using Nuxtjs, configure the plug-in:

plugins: [
    { src: '~/plugins/localStorage.js', ssr: false }
]

then, in localStorage.js:

import createPersistedState from 'vuex-persistedstate';
import * as Cookies from "js-cookie";

let cookieStorage = {
  getItem: function(key) {
    return Cookies.getJSON(key);
  },
  setItem: function(key, value) {
    return Cookies.set(key, value, {expires: 3, secure: false});
  },
  removeItem: function(key) {
    return Cookies.remove(key);
  }
};

export default (context) => {
  createPersistedState({
    storage: cookieStorage,
    getState: cookieStorage.getItem,
    setState: cookieStorage.setItem
  })(context.store);
};

2. The fetch of Nuxtjs is only used for store and cannot be used to set data. You can use AsyncData to set data, which can be used in routing pages.
III. Fetch official documents are very clear: server or before switching to the destination route
IV. Users can log in using nuxtServerInit+express session.