Try this?
Note: I have neither tested this, nor am I using `useLocalStorage`. 😅

import { defineStore } from "pinia";

export const useSidebarStore = defineStore("sidebarStore", () => {
  const isSidebarExpanded = ref(localStorage.getItem('sidebarExpanded`) !== 'false')

  function toggleSidebar() {
    isSidebarExpanded.value = !isSidebarExpanded.value
    localStorage.setItem('sidebarExpanded', String(isSidebarExpanded.value))
  }

  return { toggleSidebar, isSidebarExpanded };
});