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>

src/app/brief/infrastructure/brief.adapter.ts

@@ -249,6 +249,7 @@ function parsePassage(dto: LibraryPassageDto): Result<string, LibraryPassage> {
     content: content.value,
     version: dto.version,
     ...(dto.beroep != null ? { beroep: dto.beroep } : {}),
+    ...(dto.isDefault ? { isDefault: true } : {}),
   });
 }