fix: handle client-side routing 404 on Vercel

Add vercel.json with rewrite rules to redirect all requests to index.html.
This ensures that React Router can handle deep links and page refreshes
correctly after deployment.

vercel.json

@@ -0,0 +1,8 @@
+{
+  "rewrites": [
+    {
+      "source": "/(.*)",
+      "destination": "/index.html"
+    }
+  ]
+}