Fix Vercel build configuration and API structure

package.json

@@ -5,7 +5,7 @@
   "license": "MIT",
   "scripts": {
     "dev": "NODE_ENV=development tsx server/index.ts",
-    "build": "cd client && npm install && npm run build",
+    "build": "cd client && rm -rf node_modules package-lock.json && npm install && npm run build",
     "build:client": "cd client && npm run build",
     "build:server": "esbuild server/index.ts --platform=node --packages=external --bundle --format=esm --outdir=dist",
     "start": "NODE_ENV=production node dist/index.js",

vercel.json

@@ -1,7 +1,8 @@
 {
   "version": 2,
-  "buildCommand": "npm run build",
+  "buildCommand": "npm ci && npm run build",
   "outputDirectory": "client/dist",
+  "installCommand": "npm ci",
   "functions": {
     "api/index.ts": {
       "maxDuration": 30