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>

docs/backlog/WP-16-component-a11y.md

@@ -6,6 +6,7 @@ Phase: 4 — a11y
 ## Why
 
 Audit findings axe can't (fully) catch:
+
 - `form-field` renders a description `<div [id]="fieldId()+'-desc'">` that **no control
   ever references** — `text-input` sets `aria-describedby` only to `…-error` and only
   when invalid. Screen readers never announce field descriptions (e.g. the BSN hint on