Tag UI components and admin integration

Created by: jedmund

Summary

• Add tag components (TagPill, TagInput)
• Admin tag manager page with server-side data loading
• Integrate tag input into post metadata popover
• Display tags on post listings (admin and public universe)

Test plan

• Admin tags page loads and shows tags
• Tag input works in post editor metadata popover
• Tags display on post listings
• Search and sort work on tags page