feat(maestro): scaffold React + Tailwind + Zustand

Adds react, react-dom, zustand, tailwindcss, @tailwindcss/vite as a new
skeleton alongside the existing vanilla SPA. New src/index.tsx entry
point (unused until wired into index.html in a later issue); main.ts
stays the active entry.

web/src/App.tsx

@@ -0,0 +1,3 @@
+export function App() {
+  return <div id="react-root" />;
+}

web/src/index.tsx

@@ -0,0 +1,10 @@
+import { createRoot } from "react-dom/client";
+import { App } from "./App";
+import "./main.css";
+
+const container = document.getElementById("root");
+if (!container) {
+  throw new Error("missing #root element");
+}
+
+createRoot(container).render(<App />);

web/src/main.css

@@ -0,0 +1 @@
+@import "tailwindcss";