refactor(main): IPC 채널 SSOT 일원화 + AppConfig 타입 강화 (WS2)

- ipc-channels.ts SSOT에 9개 그룹/키 추가: INSTRUCTION, SYSTEM_AUDIO,
  POPUP_RESULT/HISTORY/COMMAND/CAPTION, VOICE_PARTIAL, CLIPBOARD, APP,
  VOICE_CONVERSATION.FINISH_LISTENING
- WindowManager/handlers/bootstrap 하드코딩 채널 -> IPC_CHANNELS 교체.
  불일치 4건(result:* vs window:tip*)은 preload 재검증 후 SSOT로 통일.
  notifyRenderer channel: string -> IPCChannel 타입 좁힘.
- AppConfig에 7개 누락 키 추가(customInstructions, llmChains,
  voiceCommandRules, voiceCommandsEnabled, activeInstructionId,
  activeChainId, captionAudioSource) -> as never 16건 제거.
- ChainService/CustomInstructionService/WindowManager dynamic require -> static import.
- services/index.ts 데드 레지스트리 제거 (import 0건).
SKIP: ipcSuccess/ipcError 헬퍼 통일, catch 패턴(별도)
정책: docs/REFACTOR_POLICY.md DP2, DP9

apps/desktop/src/main/ipc/voice-conversation-handlers.ts

@@ -131,7 +131,7 @@ export function registerVoiceConversationHandlers(): void {
   )
 
   // finishListening — 렌더러에서 녹음 종료 버튼 클릭 시
-  ipcMain.handle('voiceConversation:finishListening', async () => {
+  ipcMain.handle(IPC_CHANNELS.VOICE_CONVERSATION.FINISH_LISTENING, async () => {
     try {
       await getVoiceConversationService().finishListening()
       return ipcSuccess(undefined)