Add jp / kr / zh-TW translations and wire them into the i18n runtime so the
desktop app can switch among English, Simplified Chinese, Traditional Chinese,
Japanese, and Korean.
- new locale files jp.ts / kr.ts / zh-TW.ts with full TranslationKey coverage
- register the locales in the i18n index and extend the Locale union
- accept the new codes in settingsStore.getStoredLocale (default stays zh)
- add language-switcher entries (简体中文 / 繁體中文 / 日本語 / 한국어)
- localize chat message timestamps: Han 年月日 for zh/zh-TW/jp, Intl ko-KR for kr
- extend the DATE_LOCALES map in ActivitySettings (ja-JP / ko-KR / zh-TW)
- tests: formatMessageTimestamp locale branches + i18n locale resolution
Korean terminology follows the Microsoft Korean localization style guide
(새로 고침 / 사용·사용 안 함 / 이름 바꾸기, 합니다-style sentences).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Message rows should stay visually quiet until the user needs copy or fork controls. The action row now appears on message hover/focus, uses icon-sized controls, and shows the message timestamp beside the actions with locale-aware recent and historical formats.
Constraint: Desktop chat actions already own copy and fork affordances, so the change keeps that surface instead of adding a separate always-visible timestamp row.
Rejected: Keep text-labeled Copy buttons | the row remained too visually heavy for every message.
Confidence: high
Scope-risk: narrow
Directive: Keep copy, fork, and timestamp as one hover/focus metadata row unless the message layout is redesigned as a whole.
Tested: bun run check:desktop
Not-tested: Live Tauri native packaging
