Require Node >=20.9.0; add Nixpacks node var

Add an engines field to package.json requiring Node >=20.9.0 and add nixpacks.toml with NIXPACKS_NODE_VERSION = "20" to ensure the build/runtime use Node 20. The lockfile was updated to reflect the engine requirement.

nixpacks.toml

@@ -0,0 +1,2 @@
+[variables]
+NIXPACKS_NODE_VERSION = "20"

package-lock.json

@@ -36,6 +36,9 @@
         "eslint-config-next": "16.2.6",
         "tailwindcss": "^4",
         "typescript": "^5"
+      },
+      "engines": {
+        "node": ">=20.9.0"
       }
     },
     "node_modules/@alloc/quick-lru": {

package.json

@@ -2,6 +2,9 @@
   "name": "wfdash",
   "version": "0.1.0",
   "private": true,
+  "engines": {
+    "node": ">=20.9.0"
+  },
   "scripts": {
     "dev": "next dev --webpack",
     "build": "next build",