feat(i18n): add Korean (ko) translation

Adds a complete Korean locale (web/src/i18n/locales/ko.json) mirroring
the full key structure of en.json (1340 leaf keys, 0 missing/extra),
with interpolation placeholders and inline tags verified to match
exactly. Registers ko in the i18next resource map (i18n/config.ts)
and the language switcher dropdown (LanguageSwitcher).

Verified with:
- python key-set diff (en vs ko): 0 missing, 0 extra
- python placeholder/tag diff ({{var}}, <1>...</1>): 0 mismatches
- pnpm vitest run (full suite): 180 files / 609 tests passed
- tsc --noEmit: no errors
- eslint on changed files: no issues

web/src/i18n/config.ts

@@ -3,6 +3,7 @@ import { initReactI18next } from 'react-i18next'
 import LanguageDetector from 'i18next-browser-languagedetector'
 import en from './locales/en.json'
 import zh from './locales/zh.json'
+import ko from './locales/ko.json'
 
 /**
  * Initializes i18next for the browser app. Language preference is restored from
@@ -16,6 +17,7 @@ i18n
     resources: {
       en: { translation: en },
       zh: { translation: zh },
+      ko: { translation: ko },
     },
     fallbackLng: 'en',
     interpolation: {

web/src/shared/components/language-switcher.tsx

@@ -19,6 +19,7 @@ export function LanguageSwitcher({ className }: LanguageSwitcherProps) {
   const languages = [
     { code: 'zh', name: '中文' },
     { code: 'en', name: 'English' },
+    { code: 'ko', name: '한국어' },
   ]
 
   // 获取当前语言的主要代码（去掉地区代码）