feat(web): add Korean dashboard localization

[ovstng]

Summary

• Add Korean (ko) locale support for the web dashboard.
• Expand the dashboard language switcher to cycle through English, Chinese, and Korean.
• Detect Korean browser locales (ko, ko-*) on first load.
• Document dashboard language selection in the web dashboard guide.

Validation

• git diff --check
• npx eslint src/components/LanguageSwitcher.tsx src/i18n/context.tsx src/i18n/i18n-context.ts src/i18n/useI18n.ts src/i18n/types.ts src/i18n/en.ts src/i18n/zh.ts src/i18n/ko.ts src/i18n/index.ts
• npm run build

Note: the full npm run lint command currently fails on pre-existing baseline React lint issues outside this change; the files touched by this PR pass targeted ESLint.

Thanks for adding Korean dashboard localization. One small UX suggestion from native Korean usage: in the language switcher, it would be clearer to display Korean as 🇰🇷 한국어 instead of an abbreviation like KO, since the existing Chinese option is shown as 中文 rather than ZH.

Superseded by #22914, which has now merged. Your work on Korean directly informed this — thank you.

PR #22914 expanded i18n coverage to 16 languages (en, zh, zh-hant, ja, de, es, fr, tr, uk, af, ko, it, ga, pt, ru, hu) across the gateway slash commands, the static-message catalog, the web dashboard, and the achievements + kanban plugin dashboards.

Crediting your contribution to web dashboard (ko) in the merge commit's description. Sorry for closing rather than merging directly — your PR was stale against an updated i18n surface, but your translations and key choices were used as a reference where applicable.

Merged PR: #22914