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/renderer/components/AppLayout.tsx

@@ -15,21 +15,24 @@ import {
 import DashboardIcon from '@mui/icons-material/Dashboard'
 import HistoryIcon from '@mui/icons-material/History'
 import MenuBookIcon from '@mui/icons-material/MenuBook'
+import ExtensionIcon from '@mui/icons-material/Extension'
 import SettingsIcon from '@mui/icons-material/Settings'
 import { DashboardPage } from '../pages/DashboardPage'
 import { HistoryPage } from '../pages/HistoryPage'
 import { DictionaryPage } from '../pages/DictionaryPage'
+import { CommandsPage } from '../pages/CommandsPage'
 import { SettingsModal } from './SettingsModal'
 import { StatusBar } from './StatusBar'
 
-type Route = 'dashboard' | 'history' | 'dictionary'
+type Route = 'dashboard' | 'history' | 'dictionary' | 'commands'
 
 const DRAWER_WIDTH = 240
 
 const NAV_ITEMS: Array<{ route: Route; label: string; icon: React.ReactElement }> = [
   { route: 'dashboard', label: 'Dashboard', icon: <DashboardIcon /> },
   { route: 'history', label: 'History', icon: <HistoryIcon /> },
-  { route: 'dictionary', label: 'Dictionary', icon: <MenuBookIcon /> }
+  { route: 'dictionary', label: 'Dictionary', icon: <MenuBookIcon /> },
+  { route: 'commands', label: 'Commands', icon: <ExtensionIcon /> }
 ]
 
 export function AppLayout(): React.ReactElement {
@@ -102,6 +105,7 @@ export function AppLayout(): React.ReactElement {
         {currentRoute === 'dashboard' && <DashboardPage />}
         {currentRoute === 'history' && <HistoryPage />}
         {currentRoute === 'dictionary' && <DictionaryPage />}
+        {currentRoute === 'commands' && <CommandsPage />}
       </Box>
     </Box>
       <StatusBar />