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/placeholders.ts

@@ -77,7 +77,13 @@ function messageFor(code: DiagnosticCode, key?: string): string {
 }
 
 function diag(code: DiagnosticCode, location: DiagnosticLocation, key?: string): Diagnostic {
-  return { severity: severityOf(code), code, placeholderKey: key, location, message: messageFor(code, key) };
+  return {
+    severity: severityOf(code),
+    code,
+    placeholderKey: key,
+    location,
+    message: messageFor(code, key),
+  };
 }
 
 /**