fix: default Monaco syntax highlighting to HTML instead of plaintext

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: adrgs

frontend/src/pages/ResponseEditorPage.tsx

@@ -137,7 +137,7 @@ function getLanguage(filename: string): string {
     case "xml":
       return "xml";
     default:
-      return "plaintext";
+      return "html";
   }
 }