fix(ci): add packageManager to datum-ui package.json for reusable workflow

The reusable publish workflow (datum-cloud/actions/publish-npm-package.yaml)
reads package.json from the package path (packages/datum-ui), not the repo
root. Since that file lacked a packageManager field, pnpm/action-setup@v4
could not determine the pnpm version.

Author: yahyafakhroji

packages/datum-ui/package.json

@@ -2,6 +2,7 @@
   "name": "@datum-cloud/datum-ui",
   "type": "module",
   "version": "0.2.1",
+  "packageManager": "pnpm@10.28.1",
   "license": "MIT",
   "repository": {
     "url": "https://github.com/datum-cloud/datum-ui"