Add Japanese locale (ja) support (#1810)

* Add Japanese locale (ja) support

- New: ja.ts with 2624 UI translation keys
- New: ja.settings.ts with 1789 settings translation keys
- Registered ja in runtime.ts (Locale type, LOCALES, LOCALE_LABEL_KEYS, normalizeLocale)
- Added ja-JP BCP-47 mapping in intl.ts
- Added JA_MESSAGES in bootstrap.ts
- Added ja to lazy-load dictionary map in store.ts
- Updated messages.test.ts with ja dictionary
- Added common.language.japanese to all 8 existing locale files

* fix(i18n): complete Japanese locale updates

* test(i18n): enforce locale key parity

* docs: add Japanese documentation

---------

Co-authored-by: yuchi0531 <yuchi0531@users.noreply.github.com>
Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>

packages/ui/src/lib/i18n/messages/ko.ts

@@ -14,6 +14,7 @@ export const dict: Record<I18nKey, string> = {
   'common.language.brazilianPortuguese': '브라질 포르투갈어',
   'common.language.korean': '한국어',
   'common.language.polish': '폴란드어',
+  'common.language.japanese': '일본어',
   'common.revealPath.finder': 'Finder에서 보기',
   'common.revealPath.fileExplorer': 'File Explorer에서 열기',
   'common.revealPath.fileManager': '파일 관리자에서 열기',