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/locales.ts

@@ -1,4 +1,4 @@
-export type SupportedLocale = "ar" | "en" | "es" | "fr" | "ru" | "zh-CN";
+export type SupportedLocale = "ar" | "en" | "es" | "fr" | "ja" | "ru" | "zh-CN";
 export type AppLanguage = "system" | SupportedLocale;
 
 export interface LanguageOption {
@@ -14,16 +14,27 @@ export const LANGUAGE_OPTIONS: LanguageOption[] = [
   { value: "en", labelKey: "settings.general.language.options.en" },
   { value: "es", labelKey: "settings.general.language.options.es" },
   { value: "fr", labelKey: "settings.general.language.options.fr" },
+  { value: "ja", labelKey: "settings.general.language.options.ja" },
   { value: "ru", labelKey: "settings.general.language.options.ru" },
   { value: "zh-CN", labelKey: "settings.general.language.options.zhCN" },
 ];
 
-const SUPPORTED_LANGUAGES = new Set<AppLanguage>(["system", "ar", "en", "es", "fr", "ru", "zh-CN"]);
+const SUPPORTED_LANGUAGES = new Set<AppLanguage>([
+  "system",
+  "ar",
+  "en",
+  "es",
+  "fr",
+  "ja",
+  "ru",
+  "zh-CN",
+]);
 const LANGUAGE_NATIVE_NAMES: Record<SupportedLocale, string> = {
   ar: "العربية",
   en: "English",
   es: "Español",
   fr: "Français",
+  ja: "日本語",
   ru: "Русский",
   "zh-CN": "简体中文",
 };
@@ -33,6 +44,7 @@ const LANGUAGE_NAMES_BY_LOCALE: Record<SupportedLocale, Record<SupportedLocale,
     en: "الإنجليزية",
     es: "الإسبانية",
     fr: "الفرنسية",
+    ja: "اليابانية",
     ru: "الروسية",
     "zh-CN": "الصينية المبسطة",
   },
@@ -41,6 +53,7 @@ const LANGUAGE_NAMES_BY_LOCALE: Record<SupportedLocale, Record<SupportedLocale,
     en: "English",
     es: "Spanish",
     fr: "French",
+    ja: "Japanese",
     ru: "Russian",
     "zh-CN": "Simplified Chinese",
   },
@@ -49,6 +62,7 @@ const LANGUAGE_NAMES_BY_LOCALE: Record<SupportedLocale, Record<SupportedLocale,
     en: "inglés",
     es: "español",
     fr: "francés",
+    ja: "japonés",
     ru: "ruso",
     "zh-CN": "chino simplificado",
   },
@@ -57,14 +71,25 @@ const LANGUAGE_NAMES_BY_LOCALE: Record<SupportedLocale, Record<SupportedLocale,
     en: "anglais",
     es: "espagnol",
     fr: "français",
+    ja: "japonais",
     ru: "russe",
     "zh-CN": "chinois simplifié",
   },
+  ja: {
+    ar: "アラビア語",
+    en: "英語",
+    es: "スペイン語",
+    fr: "フランス語",
+    ja: "日本語",
+    ru: "ロシア語",
+    "zh-CN": "簡体字中国語",
+  },
   ru: {
     ar: "арабский",
     en: "английский",
     es: "испанский",
     fr: "французский",
+    ja: "японский",
     ru: "русский",
     "zh-CN": "упрощенный китайский",
   },
@@ -73,6 +98,7 @@ const LANGUAGE_NAMES_BY_LOCALE: Record<SupportedLocale, Record<SupportedLocale,
     en: "英语",
     es: "西班牙语",
     fr: "法语",
+    ja: "日语",
     ru: "俄语",
     "zh-CN": "简体中文",
   },
@@ -124,6 +150,9 @@ export function resolveSupportedLocale(
     if (normalized === "fr" || normalized.startsWith("fr-")) {
       return "fr";
     }
+    if (normalized === "ja" || normalized.startsWith("ja-")) {
+      return "ja";
+    }
     if (normalized === "ru" || normalized.startsWith("ru-")) {
       return "ru";
     }