➕ Add React-DnD dependency bundle.

frontend/vendor/react-dnd/react-dnd.js

@@ -0,0 +1,18 @@
+import { useDrag, useDrop } from "react-dnd/dist/esm/hooks";
+import { DndProvider } from "react-dnd/dist/esm/common";
+import HTML5Backend from "react-dnd-html5-backend";
+
+if (typeof self !== "undefined") { init(self); }
+else if (typeof global !== "undefined") { init(global); }
+else if (typeof window !== "undefined") { init(window); }
+else { throw new Error("unsupported execution environment"); }
+
+function init(g) {
+  g.ReactDnd = {
+    useDrag,
+    useDrop,
+    DndProvider,
+    HTML5Backend
+  };
+}
+