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/WP-24-letter-canvas.md

@@ -1,6 +1,6 @@
 # WP-24 — Letter canvas (edit on the letter)
 
-Status: todo
+Status: done
 Phase: 6 — Brief v2 (edit-on-the-letter, org templates, server-rendered preview)
 
 ## Why
@@ -63,13 +63,21 @@ domain model, `brief.machine.ts`, and every `BriefMsg` stay byte-identical.
 
 ## Acceptance criteria
 
-- [ ] Drafter edits blocks in place on the letter surface; passage picker and
+- [x] Drafter edits blocks in place on the letter surface; passage picker and
       diagnostics click-to-locate still work on the canvas.
-- [ ] Approver sees the identical surface read-only with the action bar.
-- [ ] Letterhead/footer/signature come from `orgTemplate` and are visibly non-editable.
-- [ ] `git diff` shows zero changes in `brief.machine.ts` / `brief.ts` Msg surface.
-- [ ] `letter-preview` is gone; no story regression (`test-storybook:ci` green).
-- [ ] Full GREEN + e2e smoke.
+- [x] Approver sees the identical surface read-only with the action bar.
+- [x] Letterhead/footer/signature come from `orgTemplate` and are visibly non-editable.
+- [x] `git diff` shows zero changes in `brief.machine.ts` / `brief.ts` Msg surface.
+- [x] `letter-preview` is gone; no story regression (`test-storybook:ci` green).
+- [x] Full GREEN + e2e smoke.
+
+Field notes (landed alongside): the CIBG huisstijl styles bare `<header>`/`<footer>`
+elements (robijn background), so the canvas regions are `<div>`s — the letter surface
+must stay letter.css-only. The passage picker's checkboxes now get unique
+`checkboxId`s (all previously resolved to `id="undefined"`, so labels toggled only
+the first box — multi-select was broken) and an opaque background; `.letter__body`
+stacks above the page-break marks so the dashed line never draws through content,
+while the "±pagina-einde" caption floats above everything for legibility.
 
 ## Verification