fix: use process.cwd() for public dir + simplified routes

- Changed publicDir to use process.cwd() for Vercel compatibility
- Simplified vercel.json routes: API -> server.js, everything else -> server.js
- Express.static will serve files from public/ directory

vercel.json

@@ -13,7 +13,10 @@
       "headers": {
         "Access-Control-Allow-Origin": "*"
       }
+    },
+    {
+      "src": "/(.*)",
+      "dest": "server.js"
     }
-  ],
-  "outputDirectory": "public"
+  ]
 }