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-VALIDATION.md

@@ -0,0 +1,69 @@
+---
+phase: 21
+slug: item-catalog-detail-pages
+status: draft
+nyquist_compliant: false
+wave_0_complete: false
+created: 2026-04-06
+---
+
+# Phase 21 — Validation Strategy
+
+> Per-phase validation contract for feedback sampling during execution.
+
+---
+
+## Test Infrastructure
+
+| Property | Value |
+|----------|-------|
+| **Framework** | bun test + Playwright (E2E) |
+| **Config file** | playwright.config.ts |
+| **Quick run command** | `bun run lint` |
+| **Full suite command** | `bun run lint && bun run build` |
+| **Estimated runtime** | ~10 seconds |
+
+---
+
+## Sampling Rate
+
+- **After every task commit:** Run `bun run lint`
+- **After every plan wave:** Run `bun run lint && bun run build`
+- **Before `/gsd:verify-work`:** Full suite + manual visual check
+- **Max feedback latency:** 10 seconds
+
+---
+
+## Per-Task Verification Map
+
+| Task ID | Plan | Wave | Requirement | Test Type | Automated Command | Status |
+|---------|------|------|-------------|-----------|-------------------|--------|
+| 01-T1 | 21-01 | 1 | DETAIL-01, DETAIL-03 | lint+visual | `bun run lint` | ⬜ pending |
+| 01-T2 | 21-01 | 1 | DETAIL-02 | lint+visual | `bun run lint` | ⬜ pending |
+| 02-T1 | 21-02 | 1 | DETAIL-04 | lint+visual | `bun run lint` | ⬜ pending |
+| 02-T2 | 21-02 | 1 | DETAIL-04 | lint+visual | `bun run lint` | ⬜ pending |
+| 03-T1 | 21-03 | 2 | DETAIL-04 | lint+grep | `bun run lint` | ⬜ pending |
+| 03-T2 | 21-03 | 2 | DETAIL-05 | lint+grep | `bun run lint` | ⬜ pending |
+
+---
+
+## Manual-Only Verifications
+
+| Behavior | Requirement | Why Manual | Test Instructions |
+|----------|-------------|------------|-------------------|
+| Item detail page renders with correct data | DETAIL-01 | Visual layout | Navigate to /items/:id, verify hero image, specs, notes |
+| Edit mode toggle works | DETAIL-03 | Visual interaction | Click Edit, verify fields become editable, save changes |
+| Catalog detail page with Add button | DETAIL-02 | Visual layout | Navigate to /global-items/:id, verify Add button present |
+| Candidate detail page in thread context | DETAIL-04 | Visual navigation | Click candidate in thread, verify detail page |
+| Panels removed from UI | DETAIL-05 | Visual absence | Verify no slide-out panels appear anywhere |
+
+---
+
+## Validation Sign-Off
+
+- [x] All tasks have automated verify (lint)
+- [x] Sampling continuity maintained
+- [ ] Manual visual verification pending
+- [ ] `nyquist_compliant: true` set in frontmatter
+
+**Approval:** pending