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>

packages/app/src/i18n/i18next.ts

@@ -5,6 +5,7 @@ import { ar } from "./resources/ar";
 import { en } from "./resources/en";
 import { es } from "./resources/es";
 import { fr } from "./resources/fr";
+import { ja } from "./resources/ja";
 import { ru } from "./resources/ru";
 import { zhCN } from "./resources/zh-CN";
 
@@ -20,6 +21,7 @@ observeI18nInit(
       en: { translation: en },
       es: { translation: es },
       fr: { translation: fr },
+      ja: { translation: ja },
       ru: { translation: ru },
       "zh-CN": { translation: zhCN },
     },