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

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

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

@@ -15,6 +15,7 @@ export const dict: Record<I18nKey, string> = {
   'common.language.brazilianPortuguese': 'Portugalski (Brazylia)',
   'common.language.korean': 'Koreański',
   'common.language.polish': 'Polski',
+  'common.language.japanese': 'Japoński',
   'common.revealPath.finder': 'Pokaż w Finderze',
   'common.revealPath.fileExplorer': 'Otwórz w Eksploratorze plików',
   'common.revealPath.fileManager': 'Otwórz w Menedżerze plików',