feat(fp): WP-27 — brief UX layer (undo/redo, standaardbrief, passage search, diff badges)

Brief letter-composition UX improvements:
- undo/redo history in the brief store (snapshot stacks, Ctrl/Cmd+Z) + retry-save
- "Standaardbrief invoegen" starter for empty sections; isDefault library passages
  (backend DTO/seed + adapter parse)
- passage-picker client-side search (rich-text textOf helper)
- rejection diff badges on the letter canvas + show/hide changes toggle
  (pure brief-diff domain fns + spec)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/backlog/WP-27-brief-ux-layer.md

@@ -1,6 +1,6 @@
 # WP-27 — Brief UX layer (undo/redo, standaardbrief, search, diff badges)
 
-Status: todo
+Status: done
 Phase: 6 — Brief v2 (edit-on-the-letter, org templates, server-rendered preview)
 
 ## Why
@@ -62,16 +62,16 @@ pure function over two values. Say so in code comments and stories.
 
 ## Acceptance criteria
 
-- [ ] Remove a block → Ctrl+Z restores it → Ctrl+Shift+Z re-removes; buttons mirror;
+- [x] Remove a block → Ctrl+Z restores it → Ctrl+Shift+Z re-removes; buttons mirror;
       history capped at 50; a new edit clears redo; restore re-triggers autosave.
-- [ ] Empty kern + "Standaardbrief invoegen" → default passages inserted as one undo
+- [x] Empty kern + "Standaardbrief invoegen" → default passages inserted as one undo
       step; button gone once kern is non-empty.
-- [ ] Search filters passages by label and content.
-- [ ] Reject → edit → resubmit: approver toggles "Toon wijzigingen", changed/added/
+- [x] Search filters passages by label and content.
+- [x] Reject → edit → resubmit: approver toggles "Toon wijzigingen", changed/added/
       removed blocks are badged (block granularity).
-- [ ] Autosave failure shows "Niet opgeslagen — opnieuw proberen"; retry works;
+- [x] Autosave failure shows "Niet opgeslagen — opnieuw proberen"; retry works;
       content never lost locally.
-- [ ] Full GREEN.
+- [x] Full GREEN.
 
 ## Verification