feat(i18n): add Arabic (ar-SA) language support

Add complete Arabic (Saudi Arabia) translation with 803 i18n keys, cross-validated by multiple LLMs with 15 targeted corrections applied.

Closes #427

Author: YizukiAme

lib/i18n/locales.ts

@@ -18,4 +18,5 @@ export const supportedLocales = [
   { code: 'en-US', label: 'English', shortLabel: 'EN' },
   { code: 'ja-JP', label: '日本語', shortLabel: 'JA' },
   { code: 'ru-RU', label: 'Русский', shortLabel: 'RU' },
+  { code: 'ar-SA', label: 'العربية', shortLabel: 'AR' },
 ] as const satisfies readonly LocaleEntry[];