feat: add frontend source code

Add search-zen-50 React SPA source code to frontend/ directory.
Build artifacts (dist, node_modules, lock files) are gitignored.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/main.tsx

@@ -0,0 +1,5 @@
+import { createRoot } from "react-dom/client";
+import App from "./App.tsx";
+import "./index.css";
+
+createRoot(document.getElementById("root")!).render(<App />);