feat: add toolbox contract shell and testkit

examples/react-app/src/main.tsx

@@ -0,0 +1,13 @@
+import { StrictMode } from "react";
+import { createRoot } from "react-dom/client";
+
+import { App } from "./App.js";
+
+const root = document.querySelector("#root");
+if (!root) throw new Error("Missing #root element");
+
+createRoot(root).render(
+  <StrictMode>
+    <App />
+  </StrictMode>,
+);