feat(conversation): OpenAI gpt-realtime-2.1 라이브 음성 대화 Premium 백엔드

- realtime-token Edge Function: 티어 검증 + realtime_session 쿼터 + ephemeral key 발급
- useRealtimeConversation 훅: WebRTC 직결 (마이크 → OpenAI, 오디오 자동 재생)
- VoiceConversationPage: conversationBackend 분기 + 연결 실패 시 로컬 fallback
- AppConfig.conversationBackend + SettingsModal 음성 대화 엔진 선택
- ErrorCode 799 ConversationRealtimeTokenFailed, i18n ko/en

apps/desktop/src/preload/index.ts

@@ -106,6 +106,8 @@ import type {
   ConversationAssistantDelta,
   ConversationAssistantMessage,
   ConversationError,
+  RealtimeTokenParams,
+  RealtimeTokenResult,
   // Phase 13.2
   RAGDocument,
   RAGQueryParams,
@@ -615,6 +617,11 @@ const electronAPI = {
       invoke<void>(IPC_CHANNELS.VOICE_CONVERSATION.CLEAR_HISTORY),
     cancelResponse: () =>
       invoke<void>(IPC_CHANNELS.VOICE_CONVERSATION.CANCEL_RESPONSE),
+    getRealtimeToken: (params: RealtimeTokenParams) =>
+      invoke<RealtimeTokenResult>(
+        IPC_CHANNELS.VOICE_CONVERSATION.GET_REALTIME_TOKEN,
+        params
+      ),
     finishListening: () =>
       invoke<void>('voiceConversation:finishListening'),
     onStateChanged: (cb: (data: ConversationSessionInfo) => void): Unsubscribe =>