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/domain/org-template.ts

@@ -0,0 +1,31 @@
+/**
+ * The organization template (Brief v2 PRD §3, WP-23/24): the SECOND template axis —
+ * appearance/identity per sub-organization (letterhead, footer, signature, margins).
+ * Orthogonal to the case-type template (sections + placeholders); the two only meet
+ * at render time, on the letter canvas. Server-owned: the FE renders it verbatim,
+ * never edits it here (the admin editor is WP-26).
+ */
+
+export interface Margins {
+  readonly topMm: number;
+  readonly rightMm: number;
+  readonly bottomMm: number;
+  readonly leftMm: number;
+}
+
+export interface OrgTemplate {
+  readonly subOrgId: string;
+  readonly orgName: string;
+  /** Multiline; rendered above the envelope window. */
+  readonly returnAddress: string;
+  readonly logoDocumentId?: string;
+  /** Multiline contact block in the footer. */
+  readonly footerContact: string;
+  readonly footerLegal: string;
+  readonly signatureName: string;
+  readonly signatureRole: string;
+  readonly signatureClosing: string;
+  readonly margins: Margins;
+  /** 0 = draft; n>0 = the published snapshot this letter renders with. */
+  readonly version: number;
+}