fix(ci): include pnpm workspace files in sparse checkout

The pnpm-lock.yaml and other workspace files are at the repository root,
not in frontend/. Add them to sparse checkout for pnpm install to work.

.forgejo/workflows/deploy-frontend.yml

@@ -43,6 +43,9 @@ jobs:
           sparse-checkout: |
             frontend
             schemas
+            package.json
+            pnpm-lock.yaml
+            pnpm-workspace.yaml
           sparse-checkout-cone-mode: false
 
       - name: Install pnpm