fix: use build.env in vercel.json for proper Vite build-time var injection

Author: 11KAlYAN11

frontend/vercel.json

@@ -2,11 +2,12 @@
   "buildCommand": "npm run build",
   "outputDirectory": "dist",
   "framework": "vite",
-  "env": {
-    "VITE_API_BASE_URL": "https://advtaskmanager.onrender.com/api"
+  "build": {
+    "env": {
+      "VITE_API_BASE_URL": "https://advtaskmanager.onrender.com/api"
+    }
   },
   "rewrites": [
     { "source": "/(.*)", "destination": "/index.html" }
   ]
 }
-