feat(fp): WP-24 — letter canvas (edit on the letter)

One letter surface for every role: LetterCanvasComponent renders the
org template's letterhead/signature/footer around the case-type
sections, with editableRegions content|template|none. public/letter.css
is the FE⇄BE rendering contract (WP-25 inlines it verbatim).
letter-preview deleted — its read-only rendering absorbed into 'none'
mode. brief.machine.ts byte-identical; orgTemplate parses at the
adapter boundary and lives beside the machine in BriefStore.

Also fixes passage-picker multi-select (checkboxes all shared
id="undefined", so labels only toggled the first box) and keeps the
±page-break marks from drawing through canvas content.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/app/brief/application/brief.store.spec.ts

@@ -2,6 +2,7 @@ import { TestBed } from '@angular/core/testing';
 import { describe, it, expect } from 'vitest';
 import { Result } from '@shared/kernel/fp';
 import { Brief, BriefDecisions } from '@brief/domain/brief';
+import { OrgTemplate } from '@brief/domain/org-template';
 import { BriefAdapter, BriefView } from '@brief/infrastructure/brief.adapter';
 import { BriefStore } from './brief.store';
 
@@ -22,7 +23,20 @@ const brief: Brief = {
   drafterId: 'u1',
 };
 
-const view: BriefView = { brief, availablePassages: [], decisions };
+const orgTemplate: OrgTemplate = {
+  subOrgId: 'cibg-registers',
+  orgName: 'CIBG — Registers',
+  returnAddress: 'Postbus 00000\n2500 AA Den Haag',
+  footerContact: 'info@voorbeeld.example',
+  footerLegal: 'KvK 00000000',
+  signatureName: 'A. de Vries',
+  signatureRole: 'Hoofd Registratie',
+  signatureClosing: 'Met vriendelijke groet,',
+  margins: { topMm: 25, rightMm: 25, bottomMm: 25, leftMm: 25 },
+  version: 1,
+};
+
+const view: BriefView = { brief, availablePassages: [], decisions, orgTemplate };
 
 function setup(adapter: Partial<BriefAdapter>): BriefStore {
   TestBed.configureTestingModule({ providers: [{ provide: BriefAdapter, useValue: adapter }] });