add Vercel rewrites for SPA client-side routing

All paths serve index.html so React Router handles routing.
Fixes 404 on page refresh for routes like /home, /game, etc.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vercel.json

@@ -0,0 +1,5 @@
+{
+  "rewrites": [
+    { "source": "/(.*)", "destination": "/index.html" }
+  ]
+}