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-diff.spec.ts

@@ -0,0 +1,41 @@
+import { describe, it, expect } from 'vitest';
+import { Brief, LetterBlock } from './brief';
+import { diffBlocks, changedBlocks } from './brief-diff';
+
+function block(id: string, text: string): LetterBlock {
+  return { type: 'freeText', blockId: id, content: { paragraphs: [{ nodes: [{ type: 'text', text }] }] } };
+}
+
+function brief(blocks: LetterBlock[]): Brief {
+  return {
+    briefId: 'b1',
+    beroep: 'arts',
+    templateId: 't1',
+    placeholders: [],
+    sections: [{ sectionKey: 'kern', title: 'Kern', required: true, locked: false, blocks }],
+    status: { tag: 'draft' },
+    drafterId: 'u1',
+  };
+}
+
+describe('diffBlocks', () => {
+  it('marks added, removed, changed and unchanged by blockId', () => {
+    const before = brief([block('local-1', 'a'), block('local-2', 'b'), block('local-3', 'c')]);
+    const after = brief([block('local-1', 'a'), block('local-2', 'B!'), block('local-4', 'd')]);
+    const diffs = diffBlocks(before, after);
+    const byId = new Map(diffs.map((d) => [d.blockId, d.kind]));
+    expect(byId.get('local-1')).toBe('unchanged');
+    expect(byId.get('local-2')).toBe('changed');
+    expect(byId.get('local-3')).toBe('removed'); // gone from after
+    expect(byId.get('local-4')).toBe('added'); // new in after
+  });
+
+  it('changedBlocks drops unchanged and keeps added/removed/changed', () => {
+    const before = brief([block('local-1', 'a'), block('local-2', 'b')]);
+    const after = brief([block('local-1', 'a'), block('local-2', 'B'), block('local-3', 'c')]);
+    const map = changedBlocks(diffBlocks(before, after));
+    expect(map.has('local-1')).toBe(false);
+    expect(map.get('local-2')).toBe('changed');
+    expect(map.get('local-3')).toBe('added');
+  });
+});