feat(21-01): create private item detail page with edit mode toggle

- Full detail page at /items/:id with hero image, name, spec badges, notes, product link
- Edit mode toggle: read-only by default, editable inputs when Edit clicked
- Save persists via useUpdateItem, Cancel reverts to read-only
- Duplicate and Delete actions via existing hooks/dialogs
- Back link to /collection, loading shimmer, error state
- CategoryPicker and ImageUpload in edit mode

.planning/phases/21-item-catalog-detail-pages/21-DISCUSSION-LOG.md

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+# Phase 21: Item & Catalog Detail Pages - Discussion Log
+
+> **Audit trail only.** Do not use as input to planning, research, or execution agents.
+
+**Date:** 2026-04-06
+**Phase:** 21-item-catalog-detail-pages
+**Areas discussed:** Detail page layout, Edit mode UX, Candidate navigation, Panel removal, Route structure
+**Mode:** Auto (--auto) — decisions from earlier conversation applied
+
+---
+
+## Key Decisions from Conversation
+
+| Decision | Source | Notes |
+|----------|--------|-------|
+| Private detail at `/items/:id` | User conversation | Full page, not side panel |
+| Public detail at `/global-items/:id` | User conversation | With "Add to Collection" button |
+| No visual distinction reference vs standalone | User conversation | Same layout, fields may be empty |
+| Edit via toggle button, not inline by default | User conversation | Clean aesthetic priority |
+| Candidates get detail pages too | User conversation | Same pattern as items |
+| Remove all slide-out panels | User conversation | Both item and candidate panels |
+
+## Claude's Discretion
+
+- Detail page layout proportions
+- Add Candidate pattern on thread page
+- Edit mode animation
+- Mobile adaptations
+- "Linked to catalog" indicator style
+
+## Deferred Ideas
+
+- Reviews/ratings, community stats, setup appearances, similar items, image gallery