fix: add vercel.json to handle SPA routing on Vercel

Without this configuration, page refreshes result in 404 errors because
Vercel tries to serve non-existent files instead of routing through
index.html for client-side routing.

vercel.json

@@ -0,0 +1,9 @@
+{
+  "rewrites": [
+    {
+      "source": "/(.*)",
+      "destination": "/index.html"
+    }
+  ]
+}
+