ci(ui): point pnpm/action-setup at ui/package.json

pnpm/action-setup@v4 runs with the runner's CWD (repo root) and ignores
the job-level `defaults.run.working-directory`, so the action looked
for a non-existent root `package.json` and failed with 'No pnpm
version is specified'. The pnpm version is already pinned in
ui/package.json as packageManager=pnpm@11.1.2 — this just tells the
action where to read it from.

Author: dgtalbug

.github/workflows/ci.yml

@@ -35,6 +35,11 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: pnpm/action-setup@v4
+        with:
+          # pnpm/action-setup runs with the runner CWD (repo root), not the
+          # job-level working-directory, so we point at ui/package.json
+          # explicitly. Version is read from its "packageManager" field.
+          package_json_file: ui/package.json
       - name: Set up Node 22 LTS
         uses: actions/setup-node@v4
         with: