feat(fp): WP-26 — admin org-template editor

Edit the letter's org identity in place on the same canvas the drafter composes
on (editableRegions='template'): letterhead/signature/footer become inline
controls, content a read-only sample. Margins (bounded), logo upload (reuses the
shared upload transport + single-upload), version history + rollback, proefbrief,
and publish-with-impact-confirmation. House form-machine idiom
(org-template.machine.ts) + root store with debounced save. Capability-gated
(orgtemplate:edit) with a deny-by-default alert; route /brief/huisstijl.

Backend + generated client were already in place (WP-23). Also fixes a
pre-existing red check:tokens (WP-24 canvas hex fallbacks) and threads the
published logo through to the drafter's canvas.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/backlog/README.md

@@ -70,7 +70,7 @@ for its existing violations, so every WP ends green.
 | [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-26](WP-26-org-template-editor.md)   | Admin org-template editor                                    | 6 · Brief v2                | done   |
 | [WP-27](WP-27-brief-ux-layer.md)        | Brief UX layer (undo/redo, standaardbrief, diff)             | 6 · Brief v2                | todo   |
 | [WP-28](WP-28-brief-v2-demo-polish.md)  | Brief v2 demo polish (scenarios, e2e, docs)                  | 6 · Brief v2                | todo   |