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
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