feat(i18n): add Japanese (ja) locale (#1694)

* feat(i18n): add Japanese (ja) locale

- Add packages/app/src/i18n/resources/ja.ts with 1287 keys matching en.ts
- Register ja in i18next.ts, locales.ts (SupportedLocale, LANGUAGE_OPTIONS,
  SUPPORTED_LANGUAGES, LANGUAGE_NATIVE_NAMES, LANGUAGE_NAMES_BY_LOCALE,
  resolveSupportedLocale)
- Add ja label key to all six existing locale files (ar, en, es, fr, ru, zh-CN)
- Update locales.test.ts to cover ja in all relevant assertions

Translation reviewed by Codex across four passes; terminology, placeholder
parity, and naturalness confirmed clean.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(i18n): add ja to resources.test.ts and fix test descriptions

- Import ja and add to key-parity, fallback-ratio, interpolation, and
  model-count assertions in resources.test.ts
- Update test descriptions to say "all supported language(s)" instead of
  "UN official language(s)" (ja is not a UN official language)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs: add Japanese README (README.ja.md)

Add README.ja.md translated from the English source, reviewed by Codex
across two passes. Link it in the language navigation of README.md and
README.zh-CN.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix: apply oxfmt formatting to locales.ts and ja.ts

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -6,7 +6,8 @@
 
 <p align="center">
   <a href="README.md">English</a> ·
-  <a href="README.zh-CN.md">简体中文</a>
+  <a href="README.zh-CN.md">简体中文</a> ·
+  <a href="README.ja.md">日本語</a>
 </p>
 
 <p align="center">