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/types/case.ts

@@ -102,6 +102,7 @@ type CasesMessages = {
   importCases: string;
   importAvailable: string;
   downloadTemplate: string;
+  downloadJsonSample: string;
   clickToUpload: string;
   orDragAndDrop: string;
   maxFileSize: string;