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/domain/brief.ts

@@ -25,6 +25,7 @@ export interface LibraryPassage {
   readonly label: string;
   readonly content: RichTextBlock;
   readonly version: number; // library version, for provenance only
+  readonly isDefault?: boolean; // part of the "standaardbrief" (kern) starter set
 }
 
 /** A block inside a letter section: a frozen passage snapshot, or free text. */