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>

.claude/skills/bff-endpoint/SKILL.md

@@ -34,6 +34,7 @@ server re-validates as authority).
 
    The mapping may read as an identity copy today — the point is the **types** differ,
    so the compiler forces a real mapping the moment the wire diverges.
+
 5. **Application store** exposes the resource as `RemoteData` (`fromResource`,
    combine sources with `map`/`map2`/`andThen` from `@shared/application/remote-data`).
    UI never imports infrastructure (lint-enforced).