fix: field drag uses text/plain to match DataBinding drop handler

LeftPanel was setting application/json but DataBinding reads text/plain.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/frameworks/components/layout/LeftPanel.tsx

@@ -133,10 +133,7 @@ function FieldsTab() {
   }
 
   const handleDragStart = (e: React.DragEvent, field: FieldItemVM) => {
-    e.dataTransfer.setData(
-      'application/json',
-      JSON.stringify({ fieldName: field.name, fieldType: field.type }),
-    );
+    e.dataTransfer.setData('text/plain', field.name);
     e.dataTransfer.effectAllowed = 'copy';
   };