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/phases/35-bug-fixes/35-UAT.md

@@ -0,0 +1,74 @@
+---
+status: complete
+phase: 35-bug-fixes
+source: [35-01-SUMMARY.md, 35-02-SUMMARY.md, 35-03-SUMMARY.md]
+started: 2026-04-20T00:00:00.000Z
+updated: 2026-04-20T00:01:00.000Z
+---
+
+## Current Test
+
+[testing complete]
+
+## Tests
+
+### 1. Thread Add Candidate Opens Catalog Search
+expected: On a thread detail page, clicking "Add Candidate" opens the CatalogSearchOverlay (same overlay used elsewhere), not a local modal form.
+result: pass
+
+### 2. Image Skeleton and Fade-In on Cards
+expected: On collection, thread candidates, or catalog pages, images show a gray pulsing skeleton placeholder while loading. Once the image loads, it fades in smoothly (opacity transition). Cards without images show the category icon placeholder as before.
+result: pass
+
+### 3. Login Page Redirects to OIDC
+expected: Navigating to /login immediately redirects to the Logto OIDC provider. No sign-in card or button is shown — at most a brief "Signing in..." text before the redirect.
+result: pass
+
+### 4. Cursor Pointer on Interactive Elements
+expected: Hovering over clickable ItemCards (in collection), FabMenu buttons, and BottomTabBar tab buttons shows a pointer cursor. Non-navigable ItemCards (e.g., in setup view) keep the default cursor.
+result: issue
+reported: "cursor-pointer missing on add-to-collection and thread buttons on item details page, and the small instant-add button in catalog search"
+severity: minor
+
+## Summary
+
+total: 4
+passed: 3
+issues: 1
+pending: 0
+skipped: 0
+blocked: 0
+
+## Gaps
+
+- truth: "All interactive elements show cursor-pointer on hover"
+  status: failed
+  reason: "User reported: cursor-pointer missing on add-to-collection and thread buttons on item details page, and the small instant-add button in catalog search"
+  severity: minor
+  test: 4
+  artifacts: []
+  missing: []
+
+- truth: "CatalogSearchOverlay z-index should not cover the UserMenu dropdown"
+  status: failed
+  reason: "User reported: the add candidate global search lays above the context menu which opens when clicking the avatar"
+  severity: minor
+  test: bonus
+  artifacts: []
+  missing: []
+
+- truth: "Catalog search instant-add button persists after inspecting an item and returning"
+  status: failed
+  reason: "User reported: when adding an item, you click on one to inspect it, then go back to the search, the small instant-add button isn't there"
+  severity: major
+  test: bonus
+  artifacts: []
+  missing: []
+
+- truth: "Thread creation dialog uses the CategoryPicker component"
+  status: failed
+  reason: "User reported: new thread creation dialogue doesn't use the category selector component but instead uses its own"
+  severity: minor
+  test: bonus
+  artifacts: []
+  missing: []