feat(i18n): add Korean language support (#367)

* feat(i18n): add Korean language support

- Add Korean (ko) translation files for all namespaces:
  - common.json, auth.json, settings.json, sidebar.json, chat.json, codeEditor.json
- Register Korean locale in config.js and languages.js
- Follow translation guidelines:
  - Keep technical terms in English (UI, API, Shell, Git, etc.)
  - Use Korean phonetic for some terms (TaskMaster → 테스크마스터)
  - Maintain concise translations to match English length where possible

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* fix(i18n): keep technical term "UI" in Korean translation

- Change "인터페이스" back to "UI" in sidebar subtitle
- Keep technical terms in English as per translation guidelines

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

src/i18n/locales/ko/auth.json

@@ -0,0 +1,37 @@
+{
+  "login": {
+    "title": "다시 오신 것을 환영합니다",
+    "description": "Claude Code UI 계정에 로그인하세요",
+    "username": "사용자명",
+    "password": "비밀번호",
+    "submit": "로그인",
+    "loading": "로그인 중...",
+    "errors": {
+      "invalidCredentials": "사용자명 또는 비밀번호가 잘못되었습니다",
+      "requiredFields": "모든 항목을 입력해주세요",
+      "networkError": "네트워크 오류. 다시 시도해주세요."
+    },
+    "placeholders": {
+      "username": "사용자명을 입력하세요",
+      "password": "비밀번호를 입력하세요"
+    }
+  },
+  "register": {
+    "title": "계정 생성",
+    "username": "사용자명",
+    "password": "비밀번호",
+    "confirmPassword": "비밀번호 확인",
+    "submit": "계정 생성",
+    "loading": "계정 생성 중...",
+    "errors": {
+      "passwordMismatch": "비밀번호가 일치하지 않습니다",
+      "usernameTaken": "이미 사용 중인 사용자명입니다",
+      "weakPassword": "비밀번호가 너무 약합니다"
+    }
+  },
+  "logout": {
+    "title": "로그아웃",
+    "confirm": "정말 로그아웃하시겠습니까?",
+    "button": "로그아웃"
+  }
+}