Fix Vercel configuration: use functions instead of builds to avoid conflict

vercel.json

@@ -2,12 +2,11 @@
   "version": 2,
   "buildCommand": "npm run build",
   "outputDirectory": "client/dist",
-  "builds": [
-    {
-      "src": "server/index.ts",
-      "use": "@vercel/node"
+  "functions": {
+    "server/index.ts": {
+      "maxDuration": 30
     }
-  ],
+  },
   "routes": [
     {
       "src": "/api/(.*)",
@@ -21,10 +20,5 @@
       "src": "/(.*)",
       "dest": "/index.html"
     }
-  ],
-  "functions": {
-    "server/index.ts": {
-      "maxDuration": 30
-    }
-  }
+  ]
 }