fix: sync AVAILABLE_LANGUAGES with shipped locales (add zh-CN; drop duplicate ja)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/contexts/LocalizationContext.jsx

@@ -28,7 +28,7 @@ export const AVAILABLE_LANGUAGES = [
   { code: 'nl', name: 'Dutch', nativeName: 'Nederlands' },
   { code: 'ja', name: 'Japanese', nativeName: '日本語' },
   { code: 'pt', name: 'Portuguese (Brazil)', nativeName: 'Português (Brasil)' },
-  { code: 'ja', name: 'Japanese', nativeName: '日本語' },
+  { code: 'zh-CN', name: 'Chinese (Simplified)', nativeName: '简体中文' },
 ]
 
 export const LocalizationProvider = ({ children }) => {