feat: support JSON import for test cases and add Traditional Chinese locale

JSON import lets users round-trip cases exported via the JSON download
endpoint back into a folder, alongside the existing Excel import path.

frontend/config/selection.ts

@@ -19,6 +19,7 @@ const locales: LocaleType[] = [
   { code: 'en', name: 'English' },
   { code: 'pt-BR', name: 'Português' },
   { code: 'zh-CN', name: '简体中文' },
+  { code: 'zh-TW', name: '繁體中文' },
   { code: 'ja', name: '日本語' },
 ];