Add count back to Work items sidebar nav item
## Summary
Restore the work item count display in the sidebar "Work Items" navigation item, consistent with how merge requests display their count.
## Context
From [#589202](https://gitlab.com/gitlab-org/gitlab/-/work_items/589202#note_3148676723), users have requested the return of the sidebar count that served as a quick daily activity indicator.
## Problems
- The sidebar issue count that served as a quick daily activity indicator is gone
- The counter was visible in the sidebar on every project page, not just the work item list — a saved view count doesn't replace this use case
- MRs still display their count in the sidebar, making the removal of the issue count feel inconsistent
## Acceptance Criteria
- [ ] Work Items sidebar nav item displays the count of open work items
- [ ] Count is consistent with MR count display behavior
- [ ] Count updates appropriately when work items are created/closed
## Related Issues
- [#589202](https://gitlab.com/gitlab-org/gitlab/-/work_items/589202) - Internal team feedback and issues for saved views
issue