chore: add vercel.json and Node engine pin for Vercel deploy

package.json

@@ -8,6 +8,9 @@
     "start": "next start",
     "lint": "eslint"
   },
+  "engines": {
+    "node": ">=20.0.0"
+  },
   "dependencies": {
     "@react-three/drei": "^10.7.7",
     "@react-three/fiber": "^9.6.1",

vercel.json

@@ -0,0 +1,12 @@
+{
+  "$schema": "https://openapi.vercel.sh/vercel.json",
+  "framework": "nextjs",
+  "buildCommand": "next build",
+  "installCommand": "npm install",
+  "nodeVersion": "20.x",
+  "git": {
+    "deploymentEnabled": {
+      "master": true
+    }
+  }
+}