feat: add Korean (ko) translation support

[moduvoice]

Motivation

This adds Korean (ko) translation support — to help Korean-speaking users use this
open-source project more comfortably in their own language.
(한국 사람들의 오픈소스 이용에 도움이 되게 하기 위해서 한글화 작업을 하였습니다.)

Changes

• Added messages/ko.json with a complete Korean translation of every key in messages/en.json (2341 keys, full parity), covering settings, records, chat/agent, editor, sync, and mobile UI strings.
• Added messages/common/ko.json with Korean translations for the shared common namespace (39 keys).
• Registered ko in SUPPORTED_LOCALES in src/i18n/config.ts so it is recognized by isSupportedLocale/normalizeLocale and loaded via loadMessagesWithFallback.
• Added the "한국어" option to the language selector in src/app/core/setting/general/interface-settings/language.tsx.
• Added a Korean (dayjs/locale/ko) mapping in src/components/sync-confirm-dialog.tsx so relative commit timestamps also render in Korean when the app locale is ko (mirroring the existing zh/ja/pt-BR handling).
• All ICU placeholders (e.g. {count}, {name}, {provider}) were preserved exactly. For the few plural-form strings in the English source, Korean has no grammatical plural, so they were translated as plain {count}-based strings, matching the existing convention used by ja.json/zh-TW.json for the same keys.

Testing

• Verified messages/ko.json and messages/common/ko.json are valid JSON and have exact leaf-key parity with messages/en.json / messages/common/en.json (no missing or extra keys) via a Python script.
• Verified all ICU placeholders in each string are preserved between the English source and the Korean translation.
• Ran next lint (via eslint) on the changed TypeScript files — no errors.
• Ran tsc --noEmit — no type errors.
• Ran a full next build --turbopack production build — completed successfully with all 52 routes generated, confirming the new locale files are picked up correctly by the dynamic import(\../../messages/${locale}.json`)` loader with no build-time issues.

Disclosure: this translation was produced with AI assistance (Claude) and reviewed for key parity and placeholder correctness before submitting.