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/application/brief.store.spec.ts

@@ -1,7 +1,7 @@
 import { TestBed } from '@angular/core/testing';
 import { describe, it, expect, vi, afterEach } from 'vitest';
 import { Result } from '@shared/kernel/fp';
-import { Brief, BriefDecisions } from '@brief/domain/brief';
+import { Brief, BriefDecisions, LetterBlock } from '@brief/domain/brief';
 import { OrgTemplate } from '@brief/domain/org-template';
 import { BriefAdapter, BriefView } from '@brief/infrastructure/brief.adapter';
 import { LetterPreviewAdapter } from '@brief/infrastructure/letter-preview.adapter';
@@ -105,6 +105,104 @@ describe('BriefStore action state (Idle | Busy | Failed)', () => {
   });
 });
 
+// --- WP-27: undo/redo history + rejection diff ---
+
+function block(id: string, text: string): LetterBlock {
+  return { type: 'freeText', blockId: id, content: { paragraphs: [{ nodes: [{ type: 'text', text }] }] } };
+}
+const kern = (blocks: LetterBlock[]) => ({
+  sectionKey: 'kern',
+  title: 'Kern',
+  required: true,
+  locked: false,
+  blocks,
+});
+const filledBrief: Brief = { ...brief, sections: [kern([block('local-1', 'x')])] };
+const filledView: BriefView = { ...view, brief: filledBrief };
+
+function loadedBrief(store: BriefStore): Brief {
+  const s = store.model();
+  if (s.tag !== 'loaded') throw new Error('not loaded');
+  return s.brief;
+}
+
+async function loadedStore(over: Partial<BriefAdapter> = {}): Promise<BriefStore> {
+  const ok = (v: BriefView): Promise<Result<string, BriefView>> => Promise.resolve({ ok: true, value: v });
+  const store = setup({ load: () => ok(filledView), save: () => ok(filledView), ...over });
+  await store.load();
+  return store;
+}
+
+describe('BriefStore undo/redo history', () => {
+  it('records an edit, undoes and redoes it; buttons mirror; a no-op edit is not recorded', async () => {
+    const store = await loadedStore();
+    expect(store.canUndo()).toBe(false);
+
+    store.edit({ tag: 'BlockRemoved', blockId: 'local-1' });
+    expect(loadedBrief(store).sections[0].blocks.length).toBe(0);
+    expect(store.canUndo()).toBe(true);
+
+    store.undo();
+    expect(loadedBrief(store).sections[0].blocks.length).toBe(1);
+    expect(store.canRedo()).toBe(true);
+
+    store.redo();
+    expect(loadedBrief(store).sections[0].blocks.length).toBe(0);
+
+    // A no-op edit (unknown block) changes nothing → leaves no dead history step.
+    store.undo(); // back to 1 block, redo available
+    store.edit({ tag: 'BlockRemoved', blockId: 'does-not-exist' });
+    expect(store.canRedo()).toBe(true); // future NOT cleared by a no-op
+  });
+
+  it('a new edit clears the redo future', async () => {
+    const store = await loadedStore();
+    store.edit({ tag: 'FreeTextBlockAdded', sectionKey: 'kern' });
+    store.undo();
+    expect(store.canRedo()).toBe(true);
+    store.edit({ tag: 'FreeTextBlockAdded', sectionKey: 'kern' });
+    expect(store.canRedo()).toBe(false);
+  });
+
+  it('caps history at 50 snapshots', async () => {
+    const store = await loadedStore();
+    for (let i = 0; i < 55; i++) store.edit({ tag: 'FreeTextBlockAdded', sectionKey: 'kern' });
+    let undos = 0;
+    while (store.canUndo()) {
+      store.undo();
+      undos++;
+    }
+    expect(undos).toBe(50);
+  });
+});
+
+describe('BriefStore rejection diff', () => {
+  it('captures the rejected letter and diffs a subsequent edit against it', async () => {
+    const submitted: Brief = { ...filledBrief, status: { tag: 'submitted', submittedBy: 'u', submittedAt: 't' } };
+    const rejected: Brief = {
+      ...filledBrief,
+      status: { tag: 'rejected', rejectedBy: 'u2', rejectedAt: 't', comments: 'nee' },
+    };
+    const ok = (v: BriefView): Promise<Result<string, BriefView>> => Promise.resolve({ ok: true, value: v });
+    const store = setup({
+      load: () => ok({ ...filledView, brief: submitted }),
+      save: () => ok(filledView),
+      reject: () => ok({ ...filledView, brief: rejected }),
+    });
+    await store.load();
+    await store.reject('nee');
+    expect(store.hasRejectionDiff()).toBe(false); // nothing changed yet
+
+    store.edit({
+      tag: 'BlockContentEdited',
+      blockId: 'local-1',
+      content: { paragraphs: [{ nodes: [{ type: 'text', text: 'CHANGED' }] }] },
+    });
+    expect(store.blockDiffs().get('local-1')).toBe('changed');
+    expect(store.removedSinceReject()).toBe(0);
+  });
+});
+
 describe('BriefStore.previewLetter', () => {
   // vi.spyOn reuses an existing spy (and its call history) if one is already on
   // the property — window.open/URL.createObjectURL must be restored between tests.