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>

backend/src/BigRegister.Api/Contracts/Dtos.cs

@@ -139,7 +139,7 @@ public sealed record BriefStatusDto(
 
 public sealed record LibraryPassageDto(
     string PassageId, string Scope, string SectionKey, string Label,
-    RichTextBlockDto Content, int Version, string? Beroep = null);
+    RichTextBlockDto Content, int Version, string? Beroep = null, bool IsDefault = false);
 
 public sealed record BriefDto(
     string BriefId, string Beroep, string TemplateId,