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>
+}