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>

docs/backlog/README.md

@@ -67,8 +67,8 @@ for its existing violations, so every WP ends green.
 | [WP-20](WP-20-second-locale.md)         | Second locale proof                                          | 5 · productie-volwassenheid | done   |
 | [WP-21](WP-21-resilience-seams.md)      | Resilience seams (correlation-id, idempotency, retry)        | 5 · productie-volwassenheid | done   |
 | [WP-22](WP-22-durable-persistence.md)   | Durable persistence (optional tier)                          | 5 · productie-volwassenheid | done   |
-| [WP-23](WP-23-org-template-backend.md)  | Org-template backend + admin role                            | 6 · Brief v2                | in-progress |
-| [WP-24](WP-24-letter-canvas.md)         | Letter canvas (edit on the letter)                           | 6 · Brief v2                | todo   |
+| [WP-23](WP-23-org-template-backend.md)  | Org-template backend + admin role                            | 6 · Brief v2                | done   |
+| [WP-24](WP-24-letter-canvas.md)         | Letter canvas (edit on the letter)                           | 6 · Brief v2                | done   |
 | [WP-25](WP-25-letter-preview-html.md)   | Server-rendered letter preview (HTML; PDF deferred)          | 6 · Brief v2                | todo   |
 | [WP-26](WP-26-org-template-editor.md)   | Admin org-template editor                                    | 6 · Brief v2                | todo   |
 | [WP-27](WP-27-brief-ux-layer.md)        | Brief UX layer (undo/redo, standaardbrief, diff)             | 6 · Brief v2                | todo   |