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/resources/ar.ts

@@ -1438,6 +1438,7 @@ export const ar: TranslationResources = {
           en: "English",
           es: "Español",
           fr: "Français",
+          ja: "日本語",
           ru: "Русский",
           zhCN: "中文",
         },

packages/app/src/i18n/resources/en.ts

@@ -1444,6 +1444,7 @@ export const en = {
           en: "English",
           es: "Spanish",
           fr: "French",
+          ja: "Japanese",
           ru: "Russian",
           zhCN: "Simplified Chinese",
         },

packages/app/src/i18n/resources/es.ts

@@ -1475,6 +1475,7 @@ export const es: TranslationResources = {
           en: "English",
           es: "Español",
           fr: "Français",
+          ja: "日本語",
           ru: "Русский",
           zhCN: "中文",
         },

packages/app/src/i18n/resources/fr.ts

@@ -1478,6 +1478,7 @@ export const fr: TranslationResources = {
           en: "English",
           es: "Español",
           fr: "Français",
+          ja: "日本語",
           ru: "Русский",
           zhCN: "中文",
         },

packages/app/src/i18n/resources/ru.ts

@@ -1465,6 +1465,7 @@ export const ru: TranslationResources = {
           en: "English",
           es: "Español",
           fr: "Français",
+          ja: "日本語",
           ru: "Русский",
           zhCN: "中文",
         },

packages/app/src/i18n/resources/zh-CN.ts

@@ -1420,6 +1420,7 @@ export const zhCN: TranslationResources = {
           en: "English",
           es: "Español",
           fr: "Français",
+          ja: "日本語",
           ru: "Русский",
           zhCN: "简体中文",
         },