Pin pnpm version in packageManager field to fix Docker build

corepack prepare pnpm --activate without a version was resolving to
12.0.0-alpha.0 (broken pre-release), causing the dockerhub-build-push
job to fail. Adding packageManager field pins it to a stable release.

ui/package.json

@@ -21,6 +21,7 @@
       }
     ]
   },
+  "packageManager": "pnpm@11.5.2",
   "private": true,
   "dependencies": {
     "@angular/animations": "^22.0.4",