Redesign admin tags page to list layout

Created by: jedmund

Summary

• Replace card grid with list layout matching other admin pages
• Add custom Checkbox component (red selected state, SVG checkmark)
• Add TagListItem with three-dot dropdown menu (edit/delete)
• Use AdminFilters with small/minimal sort dropdown
• Use DeleteConfirmationModal instead of browser confirm()
• Fix Select component medium size to match Input height
• Sentence case on action buttons across admin pages

Test plan

• Tags page renders as a list, not a grid
• Checkbox selection works for merge
• Merge button only appears with 2+ tags selected
• Edit/delete via dropdown menus work
• Delete shows confirmation modal
• Sort dropdown options all work
• Search still filters correctly