✨(frontend) add localStorage persistence for audio output device

Implement audio output device persistence in localStorage since LiveKit
doesn't handle this by default.

Ensures user's preferred audio output selection is remembered across
sessions, improving user experience by maintaining device preferences
without requiring re-selection on each visit.

src/frontend/src/features/rooms/livekit/hooks/usePersistentUserChoices.ts

@@ -16,6 +16,9 @@ export function usePersistentUserChoices() {
     saveAudioInputDeviceId: (deviceId: string) => {
       userChoicesStore.audioDeviceId = deviceId
     },
+    saveAudioOutputDeviceId: (deviceId: string) => {
+      userChoicesStore.audioOutputDeviceId = deviceId
+    },
     saveVideoInputDeviceId: (deviceId: string) => {
       userChoicesStore.videoDeviceId = deviceId
     },

src/frontend/src/stores/userChoices.ts

@@ -9,11 +9,13 @@ import {
 export type LocalUserChoices = LocalUserChoicesLK & {
   processorSerialized?: ProcessorSerialized
   noiseReductionEnabled?: boolean
+  audioOutputDeviceId?: string
 }
 
 function getUserChoicesState(): LocalUserChoices {
   return {
     noiseReductionEnabled: false,
+    audioOutputDeviceId: 'default', // Use 'default' to match LiveKit's standard device selection behavior
     ...loadUserChoices(),
   }
 }