Add Vercel SPA routing config to fix 404 on direct navigation

Vercel returns 404 when navigating directly to routes like /run/:slug
because it looks for a matching file on disk. This rewrite rule tells
Vercel to serve index.html for all routes, letting React Router handle
client-side routing. Equivalent to the existing Netlify _redirects file.

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

Author: bfirsh

web/vercel.json

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