refactor: repartition server-side and client-side code

app/.client/stores/sidebar.ts

@@ -0,0 +1,15 @@
+import { atom } from 'nanostores';
+
+export const kSidebar = 'upage_sidebar';
+
+export const DEFAULT_SIDEBAR_STATE = false;
+
+export const sidebarStore = atom<boolean>(DEFAULT_SIDEBAR_STATE);
+
+export function toggleSidebar() {
+  const currentSidebar = sidebarStore.get();
+  const newSidebar = !currentSidebar;
+
+  // Update the theme store
+  sidebarStore.set(newSidebar);
+}