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/languages.js

@@ -14,6 +14,11 @@ export const languages = [
     label: 'English',
     nativeName: 'English',
   },
+  {
+    value: 'ko',
+    label: 'Korean',
+    nativeName: '한국어',
+  },
   {
     value: 'zh-CN',
     label: 'Simplified Chinese',