docs(21-01): complete detail pages plan — item detail with edit mode, catalog Add button

- SUMMARY.md with task commits and known stubs
- STATE.md updated to phase 21, plan 1 of 3 complete
- ROADMAP.md updated with plan progress

.planning/ROADMAP.md

@@ -266,6 +266,6 @@ Plans:
 | 18. Global Items & Public Profiles | v2.0 | 4/5 | Complete    | 2026-04-05 |
 | 19. Reference Item Model & Tags Schema | v2.0 | 3/3 | Complete    | 2026-04-05 |
 | 20. FAB & Full-Screen Catalog Search | v2.0 | 2/2 | Complete    | 2026-04-06 |
-| 21. Item & Catalog Detail Pages | v2.0 | 0/? | Not started | - |
+| 21. Item & Catalog Detail Pages | v2.0 | 1/3 | In Progress|  |
 | 22. Add-from-Catalog & Thread Integration | v2.0 | 0/? | Not started | - |
 | 23. Manual Entry Fallback | v2.0 | 0/? | Not started | - |