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/shared/infrastructure/api-client.ts

@@ -1712,6 +1712,7 @@ export interface LibraryPassageDto {
     content?: RichTextBlockDto;
     version?: number;
     beroep?: string | undefined;
+    isDefault?: boolean;
 }
 
 export interface ManualDiplomaPolicyDto {

src/app/shared/kernel/rich-text.ts

@@ -49,6 +49,15 @@ export function deepCopyBlock(block: RichTextBlock): RichTextBlock {
   return structuredClone(block) as RichTextBlock;
 }
 
+/** All visible text of a block as one lowercased string — for client-side search over
+    passages. Placeholders contribute their key so "naam" matches a `naam_zorgverlener` chip. */
+export function textOf(block: RichTextBlock): string {
+  return block.paragraphs
+    .flatMap((p) => p.nodes.map((n) => (n.type === 'text' ? n.text : n.type === 'placeholder' ? n.key : '')))
+    .join(' ')
+    .toLowerCase();
+}
+
 /** Every placeholder key used in a block, in document order (duplicates kept — the
     caller dedupes when it wants a set). */
 export function placeholderKeysIn(block: RichTextBlock): string[] {