fix(admin): move detail routes to directory structure to fix rendering

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>

.planning/ROADMAP.md

@@ -401,3 +401,11 @@ Plans:
 
 Plans:
 - [ ] TBD (promote with /gsd:review-backlog when ready)
+
+### Phase 999.12: Admin UX Polish (BACKLOG)
+**Goal**: Overhaul admin panel UX with TanStack Table (sortable/groupable columns) and cmdk (GitLab-style composable filter bar with field→operator→value token input). Hide FAB on /admin/* pages. Replace tag inline form with popup modal. Show tags expanded on item rows (collapse to +N when tight). Group items by brand. Prominent search bar on both admin list pages.
+**Requirements**: TBD
+**Plans**: TBD
+
+Plans:
+- [ ] TBD (promote with /gsd:review-backlog when ready)