style: format frontend, docs and skills with prettier; add .prettierignore

One-time prettier --write so the new format:check CI gate starts green.
.prettierignore excludes generated (api-client.ts, documentation.json),
vendored (public/cibg-huisstijl), and backend (dotnet format owns it).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app/brief/domain/brief.ts

@@ -59,7 +59,12 @@ export type BriefStatus =
   | { readonly tag: 'draft' }
   | { readonly tag: 'submitted'; readonly submittedBy: string; readonly submittedAt: string }
   | { readonly tag: 'approved'; readonly approvedBy: string; readonly approvedAt: string }
-  | { readonly tag: 'rejected'; readonly rejectedBy: string; readonly rejectedAt: string; readonly comments: string }
+  | {
+      readonly tag: 'rejected';
+      readonly rejectedBy: string;
+      readonly rejectedAt: string;
+      readonly comments: string;
+    }
   | { readonly tag: 'sent'; readonly sentAt: string };
 
 export interface Brief {
@@ -81,7 +86,9 @@ export function allBlocks(brief: Brief): LetterBlock[] {
 /** Every diagnostic in the letter, in section→block→node order. This is what the
     diagnostics panel renders and what the send gate checks. */
 export function allDiagnostics(brief: Brief): Diagnostic[] {
-  return allBlocks(brief).flatMap((b) => lintPlaceholders(b.content, brief.placeholders, b.blockId));
+  return allBlocks(brief).flatMap((b) =>
+    lintPlaceholders(b.content, brief.placeholders, b.blockId),
+  );
 }
 
 export function hasBlockingErrors(diagnostics: readonly Diagnostic[]): boolean {