Add loading skeletons to list pages

## Description

Replace generic \"Loading...\" text with skeleton loading animations on list pages for a better user experience. ## Current Behavior

Pages show \"Loading...\" text while data is being fetched. ## Expected Behavior

Show animated skeleton placeholders that match the layout of the content being loaded (table rows, cards, etc.). ## Pages to Update

• [ ] Items list
• [ ] Customers list
• [ ] Sales list
• [ ] Work orders list
• [ ] Dashboard cards ## Implementation
• Create a reusable \