build: Add minimum node version warning

An issue was found when using Node version 24.9 with the latest
package-lock (see #123), one not reproducible with versions 24.12 and
above.  Add a warning that will be shown when running `npm ci`.

Author: arbrandes

package-lock.json

@@ -2,6 +2,9 @@
   "name": "@openedx/frontend-app-authn",
   "version": "1.0.0-alpha.4",
   "description": "Frontend authentication",
+  "engines": {
+    "node": "^24.12"
+  },
   "repository": {
     "type": "git",
     "url": "git+https://github.com/openedx/frontend-app-authn.git"