client/web: pipe through to React in dev mode

Updates tailscale/corp#13775

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

client/web/src/components/app.tsx

@@ -0,0 +1,5 @@
+import React from "react"
+
+export default function App() {
+  return <div>Hello world</div>
+}

client/web/src/index.tsx

@@ -0,0 +1,16 @@
+import React from "react"
+import { createRoot } from "react-dom/client"
+import App from "./components/app"
+
+const rootEl = document.createElement("div")
+rootEl.id = "app-root"
+rootEl.classList.add("relative", "z-0")
+document.body.append(rootEl)
+
+const root = createRoot(rootEl)
+
+root.render(
+  <React.StrictMode>
+    <App />
+  </React.StrictMode>
+)