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/caption-handlers.ts

@@ -9,7 +9,7 @@ import type { CaptionConfig } from '@d3ro/core/types'
 
 export function registerCaptionHandlers(): void {
   // 시스템 오디오 루프백: setDisplayMediaRequestHandler로 audio: 'loopback' 설정
-  ipcMain.handle('system-audio:enable-loopback', async () => {
+  ipcMain.handle(IPC_CHANNELS.SYSTEM_AUDIO.ENABLE_LOOPBACK, async () => {
     session.defaultSession.setDisplayMediaRequestHandler(async (_request, callback) => {
       const sources = await desktopCapturer.getSources({ types: ['screen'] })
       if (sources.length === 0) {
@@ -21,7 +21,7 @@ export function registerCaptionHandlers(): void {
     return ipcSuccess(undefined)
   })
 
-  ipcMain.handle('system-audio:disable-loopback', async () => {
+  ipcMain.handle(IPC_CHANNELS.SYSTEM_AUDIO.DISABLE_LOOPBACK, async () => {
     session.defaultSession.setDisplayMediaRequestHandler(null)
     return ipcSuccess(undefined)
   })