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/ui/letter-canvas/letter-canvas.stories.ts

@@ -0,0 +1,154 @@
+import type { Meta, StoryObj } from '@storybook/angular';
+import { Brief, LibraryPassage, allDiagnostics } from '@brief/domain/brief';
+import { OrgTemplate } from '@brief/domain/org-template';
+import { LetterCanvasComponent } from './letter-canvas.component';
+
+const orgTemplate: OrgTemplate = {
+  subOrgId: 'cibg-registers',
+  orgName: 'CIBG — Registers',
+  returnAddress: 'BIG-register\nPostbus 00000\n2500 AA Den Haag',
+  footerContact: 'www.bigregister.nl\ninfo@voorbeeld.example\n070 000 00 00',
+  footerLegal: 'Ons kenmerk vermelden bij correspondentie.',
+  signatureName: 'A. de Vries',
+  signatureRole: 'Hoofd Registratie',
+  signatureClosing: 'Met vriendelijke groet,',
+  margins: { topMm: 25, rightMm: 25, bottomMm: 25, leftMm: 25 },
+  version: 1,
+};
+
+const passages: LibraryPassage[] = [
+  {
+    passageId: 'p1',
+    scope: 'global',
+    sectionKey: 'kern',
+    label: 'Standaard toelichting',
+    version: 1,
+    content: { paragraphs: [{ nodes: [{ type: 'text', text: 'Standaardtekst.' }] }] },
+  },
+];
+
+const brief: Brief = {
+  briefId: 'b1',
+  beroep: 'arts',
+  templateId: 't1',
+  drafterId: 'demo-drafter',
+  status: { tag: 'draft' },
+  placeholders: [
+    { key: 'naam_zorgverlener', label: 'Naam zorgverlener', autoResolvable: true },
+    { key: 'reden_besluit', label: 'Reden besluit', autoResolvable: false },
+  ],
+  sections: [
+    {
+      sectionKey: 'aanhef',
+      title: 'Aanhef',
+      required: true,
+      locked: true,
+      blocks: [
+        {
+          type: 'passage',
+          blockId: 'local-1',
+          sourcePassageId: 'p1',
+          sourceVersion: 1,
+          edited: false,
+          content: {
+            paragraphs: [
+              {
+                nodes: [
+                  { type: 'text', text: 'Geachte heer/mevrouw ' },
+                  { type: 'placeholder', key: 'naam_zorgverlener' },
+                  { type: 'text', text: ',' },
+                ],
+              },
+            ],
+          },
+        },
+      ],
+    },
+    {
+      sectionKey: 'kern',
+      title: 'Kern van het besluit',
+      required: true,
+      locked: false,
+      blocks: [
+        {
+          type: 'freeText',
+          blockId: 'local-2',
+          content: {
+            paragraphs: [
+              {
+                nodes: [
+                  { type: 'text', text: 'Wij hebben besloten om reden ' },
+                  { type: 'placeholder', key: 'reden_besluit' },
+                  { type: 'text', text: '.' },
+                ],
+              },
+            ],
+          },
+        },
+      ],
+    },
+  ],
+};
+
+/** Enough repeated body text to push the surface past one A4 page. */
+const longBrief: Brief = {
+  ...brief,
+  sections: brief.sections.map((s) =>
+    s.sectionKey === 'kern'
+      ? {
+          ...s,
+          blocks: [
+            {
+              type: 'freeText',
+              blockId: 'local-long',
+              content: {
+                paragraphs: Array.from({ length: 40 }, (_, i) => ({
+                  nodes: [
+                    {
+                      type: 'text' as const,
+                      text: `Alinea ${i + 1}: de beoordeling van uw aanvraag is uitgevoerd volgens de geldende regels voor herregistratie in het BIG-register.`,
+                    },
+                  ],
+                })),
+              },
+            },
+          ],
+        }
+      : s,
+  ),
+};
+
+const meta: Meta<LetterCanvasComponent> = {
+  title: 'Domein/Brief/Letter Canvas',
+  component: LetterCanvasComponent,
+  args: {
+    brief,
+    orgTemplate,
+    availablePassages: passages,
+    placeholders: brief.placeholders,
+    diagnostics: allDiagnostics(brief),
+  },
+};
+export default meta;
+type Story = StoryObj<LetterCanvasComponent>;
+
+/** Drafter: content blocks editable in place; org-identity regions tinted read-only. */
+export const ContentMode: Story = { args: { editableRegions: 'content' } };
+
+/** Approver/locked: the identical surface fully read-only, with the sample-values
+    toggle and diagnostic placeholder chips (absorbs the old Letter Preview). */
+export const ReadOnly: Story = { args: { editableRegions: 'none' } };
+
+export const ReadOnlyZonderBevindingen: Story = {
+  args: { editableRegions: 'none', diagnostics: [] },
+};
+
+/** Admin editor focus (consumer arrives in WP-26): body read-only, no "not yours" tint. */
+export const TemplateMode: Story = { args: { editableRegions: 'template' } };
+
+export const Zoomed: Story = { args: { editableRegions: 'none', zoom: 0.6 } };
+
+/** Long letter: the approximate ±page-break marks appear per A4 interval. */
+export const PageBreak: Story = {
+  args: { editableRegions: 'none', brief: longBrief, diagnostics: [] },
+};