client/web: add tailwind styling to react app

Updates tailscale/corp#13775

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

client/web/src/components/app.tsx

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