✨(frontend) add localStorage persistence for user preference settings

Persist user preference choices across sessions using localStorage
following notification store pattern, eliminating need to reconfigure
disabled features on every meeting join and respecting user's
long-term preference decisions.

src/frontend/src/stores/userPreferences.ts

@@ -1,9 +1,38 @@
 import { proxy } from 'valtio'
+import { subscribe } from 'valtio/index'
+import { STORAGE_KEYS } from '@/utils/storageKeys'
 
 type State = {
   is_idle_disconnect_modal_enabled: boolean
 }
 
-export const userPreferencesStore = proxy<State>({
+const DEFAULT_STATE = {
   is_idle_disconnect_modal_enabled: true,
+}
+
+function getUserPreferencesState(): State {
+  try {
+    const stored = localStorage.getItem(STORAGE_KEYS.USER_PREFERENCES)
+    if (!stored) return DEFAULT_STATE
+    const parsed = JSON.parse(stored)
+    return {
+      ...DEFAULT_STATE,
+      ...parsed,
+    }
+  } catch (error: unknown) {
+    console.error(
+      '[UserPreferencesStore] Failed to parse stored settings:',
+      error
+    )
+    return DEFAULT_STATE
+  }
+}
+
+export const userPreferencesStore = proxy<State>(getUserPreferencesState())
+
+subscribe(userPreferencesStore, () => {
+  localStorage.setItem(
+    STORAGE_KEYS.USER_PREFERENCES,
+    JSON.stringify(userPreferencesStore)
+  )
 })

src/frontend/src/utils/storageKeys.tsx

@@ -3,4 +3,5 @@
  */
 export const STORAGE_KEYS = {
   NOTIFICATIONS: 'app_notification_settings',
+  USER_PREFERENCES: 'app_user_preferences',
 } as const