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');
+  });
+});

src/app/brief/domain/brief-diff.ts

@@ -0,0 +1,50 @@
+import { Brief, LetterBlock, allBlocks } from './brief';
+
+/**
+ * The rejection diff as a PURE function over two immutable `Brief` values — the whole
+ * teaching payload of WP-27: because state is one value, "what changed since the letter
+ * was rejected" is just a fold over two snapshots, no change-tracking bookkeeping.
+ *
+ * Blocks are matched by `blockId` (stable `local-N`/seed ids):
+ *  - in `after` but not `before` → `added`
+ *  - in `before` but not `after` → `removed`
+ *  - in both, different content   → `changed`
+ *  - in both, same content        → `unchanged`
+ */
+
+export type BlockDiffKind = 'added' | 'removed' | 'changed' | 'unchanged';
+
+export interface BlockDiff {
+  readonly blockId: string;
+  readonly kind: BlockDiffKind;
+}
+
+/** Content equality by value. Blocks are JSON-shaped immutable trees, so a canonical
+    stringify is an honest deep-equal here (no functions, no cycles). */
+function contentEqual(a: LetterBlock, b: LetterBlock): boolean {
+  return JSON.stringify(a.content) === JSON.stringify(b.content);
+}
+
+export function diffBlocks(before: Brief, after: Brief): BlockDiff[] {
+  const beforeById = new Map(allBlocks(before).map((b) => [b.blockId, b]));
+  const afterById = new Map(allBlocks(after).map((b) => [b.blockId, b]));
+  const out: BlockDiff[] = [];
+  for (const a of afterById.values()) {
+    const b = beforeById.get(a.blockId);
+    out.push({
+      blockId: a.blockId,
+      kind: !b ? 'added' : contentEqual(a, b) ? 'unchanged' : 'changed',
+    });
+  }
+  for (const b of beforeById.values()) {
+    if (!afterById.has(b.blockId)) out.push({ blockId: b.blockId, kind: 'removed' });
+  }
+  return out;
+}
+
+/** Lookup of only the blocks that changed since rejection (drops `unchanged`), for
+    badging the canvas. Keyed by `blockId`; removed ids are present too (the caller
+    surfaces them as a count — a removed block no longer renders inline). */
+export function changedBlocks(diffs: readonly BlockDiff[]): ReadonlyMap<string, BlockDiffKind> {
+  return new Map(diffs.filter((d) => d.kind !== 'unchanged').map((d) => [d.blockId, d.kind]));
+}

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. */