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>

.storybook/preview-head.html

@@ -1,3 +1,5 @@
 <!-- CIBG Huisstijl (customized Bootstrap 5.2), served from the vendored public/ staticDir.
      Mirrors src/index.html so stories match the app. System font per styles.scss. -->
 <link rel="stylesheet" href="cibg-huisstijl/css/huisstijl.min.css" />
+<!-- Letter-rendering contract (WP-24), mirrors index.html. -->
+<link rel="stylesheet" href="letter.css" />