Admin item/tag edit pages weren't rendering because TanStack Router treated
them as children of the list route (which had no Outlet). Moving to
directory-based routing (items/index.tsx + items/$itemId.tsx) makes them
siblings that render directly in the admin layout.
Also adds UAT results for phases 35-38 and backlog item 999.12 (Admin UX Polish).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
