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/auth/infrastructure/digid.adapter.ts

@@ -9,7 +9,8 @@ export class DigidAdapter {
   // Swap for a real OIDC redirect flow when there's a backend.
   async authenticate(bsn: string): Promise<Result<string, Session>> {
     const t = bsn.trim();
-    if (!/^\d{9}$/.test(t)) return err($localize`:@@validation.bsn:Voer een geldig BSN van 9 cijfers in.`);
+    if (!/^\d{9}$/.test(t))
+      return err($localize`:@@validation.bsn:Voer een geldig BSN van 9 cijfers in.`);
     return ok({ bsn: t, naam: 'Dr. A. (Anna) de Vries' });
   }
 }