enh(pagination): add loading guard, total count, jump to page

[benjaminfrueh]

Summary

Improves the pagination in the admin settings UI, making it usable for instances with a large number of team folders.

Changes

Backend

• Add countAllFolders() to FolderManager to get the total folder count
• Add GET /folders/count endpoint in FolderController
• Add \d+ requirement to GET /folders/{id} to prevent route conflict with /folders/count

Frontend

• Add loading guard to goToPage to prevent duplicate entries on clicks before loading finishes
• Fetch total folder count to enable proper last-page calculation
• Fix pagination buttons to use total count instead of loaded folders length
• Add input to switch to a page (shown only when there are more than 5 pages)

Screenshot