Phase 6 구현: 커스텀 명령어 + i18n (ko/en)

- CustomInstructionService: 프리셋 5개 (번역/요약/전문리라이트/코드설명/자유프롬프트)
- 커스텀 명령어 CRUD + 프리셋 보호 (삭제 불가)
- CommandsPage: 명령어 목록 + 추가/편집 다이얼로그
- i18n: ko.json/en.json 리소스 파일, t() 함수, React 컨텍스트
- IPC: instruction 핸들러 6개
- AppLayout: Commands 네비게이션 추가

src/main/ipc/instruction-handlers.ts

@@ -0,0 +1,67 @@
+// src/main/ipc/instruction-handlers.ts
+
+import { ipcMain } from 'electron'
+import { ipcSuccess, ipcError, ErrorCode } from '@shared/errors'
+import { getCustomInstructionService } from '../services/CustomInstructionService'
+import type { CustomInstruction } from '../services/CustomInstructionService'
+
+// IPC_CHANNELS에 instruction 채널이 없으므로 직접 문자열 사용
+// (Phase 6 전용, 설계서 02에는 미포함)
+const CHANNELS = {
+  GET_ALL: 'instruction:getAll',
+  GET_BY_ID: 'instruction:getById',
+  CREATE: 'instruction:create',
+  UPDATE: 'instruction:update',
+  DELETE: 'instruction:delete',
+  REORDER: 'instruction:reorder'
+} as const
+
+export function registerInstructionHandlers(): void {
+  ipcMain.handle(CHANNELS.GET_ALL, async () => {
+    return ipcSuccess(getCustomInstructionService().getAll())
+  })
+
+  ipcMain.handle(CHANNELS.GET_BY_ID, async (_event, params: { id: string }) => {
+    return ipcSuccess(getCustomInstructionService().getById(params.id))
+  })
+
+  ipcMain.handle(
+    CHANNELS.CREATE,
+    async (
+      _event,
+      params: { name: string; description: string; prompt: string; icon?: string }
+    ) => {
+      try {
+        const result = getCustomInstructionService().create(params)
+        return ipcSuccess(result)
+      } catch {
+        return ipcError(ErrorCode.ConfigWriteFailed, 'Failed to create instruction')
+      }
+    }
+  )
+
+  ipcMain.handle(
+    CHANNELS.UPDATE,
+    async (_event, params: { id: string; data: Partial<CustomInstruction> }) => {
+      try {
+        const result = getCustomInstructionService().update(params.id, params.data)
+        return ipcSuccess(result)
+      } catch {
+        return ipcError(ErrorCode.ConfigWriteFailed, 'Failed to update instruction')
+      }
+    }
+  )
+
+  ipcMain.handle(CHANNELS.DELETE, async (_event, params: { id: string }) => {
+    const result = getCustomInstructionService().delete(params.id)
+    if (!result) {
+      return ipcError(ErrorCode.ConfigWriteFailed, 'Cannot delete builtin instruction')
+    }
+    return ipcSuccess(undefined)
+  })
+
+  ipcMain.handle(CHANNELS.REORDER, async (_event, params: { ids: string[] }) => {
+    getCustomInstructionService().reorder(params.ids)
+    return ipcSuccess(undefined)
+  })
+}