feat(brief): letter composition + two-person approval (teaching slice)

New `brief` context — a letter-composition feature with a drafter/approver
approval workflow, built as a teaching vertical slice on the repo's existing
FP + Elm + atomic-design patterns (see plan in ~/.claude/plans).

Domain (pure):
- Rich text as a serialisable value tree (placeholders are first-class nodes),
  moved to @shared/kernel/rich-text.ts so the shared editor can use it.
- lintPlaceholders: a pure, total content -> Diagnostic[] linter, derived never stored.
- brief.machine.ts: status sum-type with guarded transitions; frozen-snapshot =
  deep value copy; derived diagnostics/editability. Full specs.

Backend (.NET stub):
- BriefStore + seed, GET/PUT /brief and submit/approve/reject/send endpoints,
  role via X-Role header (mirrors X-Admin), transition + approver!=drafter guards,
  audit logging. Regenerated typed client via gen:api. +6 backend tests.

Seam:
- brief.adapter.ts maps flat wire unions <-> domain discriminated unions at the
  parse boundary (+ spec).

UI (atomic):
- shared atoms: checkbox, placeholder-chip; molecule: rich-text-editor (no-dep
  contenteditable, DOM<->RichTextBlock round-trip tested).
- brief/ui: letter-block, passage-picker, diagnostics-panel, rejection-comments,
  letter-section, letter-composer, letter-preview, brief.page + /brief route.
- Dev-only ?role=drafter|approver toggle + roleInterceptor; dashboard nav link.

Enforcement: @brief/* alias + eslint layer boundary (brief depends only on shared).

Also included (same session):
- Value-object specs (postcode/uren/big-nummer) — closes the "domain must have a spec" gap.
- src/docs/ Storybook MDX foundation pages (atomic design, tokens, FP-in-UI).
- .storybook/tsconfig.json: add @angular/localize to types (Storybook was fully
  broken — $localize unresolved — dev + build).

Verified: 168 FE tests, 68 backend tests, lint/build/check:tokens green,
Storybook boots, end-to-end HTTP smoke (self-approve 403, approver 200, full flow).

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

backend/src/BigRegister.Api/Contracts/Dtos.cs

@@ -107,3 +107,45 @@ public sealed record SubmitApplicationRequest(
     IReadOnlyList<DocumentRefDto>? Documents = null);
 
 public sealed record SubmitApplicationResponse(string Referentie, AanvraagStatusDto Status);
+
+// --- Brief (letter composition) contracts ---
+// Rich text is a serialisable node tree; the node union is flattened with a `Type`
+// discriminator + nullable fields, the same wire convention as AanvraagStatusDto.
+// The backend treats the content as opaque (stores/returns it) — the FE owns the
+// rich-text semantics and re-validates at its parse* boundary.
+
+public sealed record RichTextNodeDto(string Type, string? Text = null, IReadOnlyList<string>? Marks = null, string? Key = null);
+public sealed record ParagraphDto(IReadOnlyList<RichTextNodeDto> Nodes);
+public sealed record RichTextBlockDto(IReadOnlyList<ParagraphDto> Paragraphs);
+
+public sealed record PlaceholderDefDto(string Key, string Label, bool AutoResolvable, bool? Fillable = null, bool? Deprecated = null);
+
+// LetterBlock union (passage | freeText), flattened: passage-only fields are nullable.
+public sealed record LetterBlockDto(
+    string Type, string BlockId, RichTextBlockDto Content,
+    string? SourcePassageId = null, int? SourceVersion = null, bool? Edited = null);
+
+public sealed record LetterSectionDto(string SectionKey, string Title, bool Required, IReadOnlyList<LetterBlockDto> Blocks);
+
+// BriefStatus union, flattened by Tag (draft | submitted | approved | rejected | sent).
+public sealed record BriefStatusDto(
+    string Tag,
+    string? SubmittedBy = null, string? SubmittedAt = null,
+    string? ApprovedBy = null, string? ApprovedAt = null,
+    string? RejectedBy = null, string? RejectedAt = null, string? Comments = null,
+    string? SentAt = null);
+
+public sealed record LibraryPassageDto(
+    string PassageId, string Scope, string SectionKey, string Label,
+    RichTextBlockDto Content, int Version, string? Beroep = null);
+
+public sealed record BriefDto(
+    string BriefId, string Beroep, string TemplateId,
+    IReadOnlyList<PlaceholderDefDto> Placeholders,
+    IReadOnlyList<LetterSectionDto> Sections,
+    BriefStatusDto Status, string DrafterId);
+
+public sealed record BriefViewDto(BriefDto Brief, IReadOnlyList<LibraryPassageDto> AvailablePassages);
+
+public sealed record SaveBriefRequest(IReadOnlyList<LetterSectionDto> Sections);
+public sealed record RejectBriefRequest(string Comments);