refactor: repartition server-side and client-side code

app/.client/stores/profile.ts

@@ -0,0 +1,28 @@
+import { atom } from 'nanostores';
+
+interface Profile {
+  username: string;
+  bio: string;
+  avatar: string;
+}
+
+// Initialize with stored profile or defaults
+const storedProfile = typeof window !== 'undefined' ? localStorage.getItem('upage_profile') : null;
+const initialProfile: Profile = storedProfile
+  ? JSON.parse(storedProfile)
+  : {
+      username: '',
+      bio: '',
+      avatar: '',
+    };
+
+export const profileStore = atom<Profile>(initialProfile);
+
+export const updateProfile = (updates: Partial<Profile>) => {
+  profileStore.set({ ...profileStore.get(), ...updates });
+
+  // Persist to localStorage
+  if (typeof window !== 'undefined') {
+    localStorage.setItem('upage_profile', JSON.stringify(profileStore.get()));
+  }
+};