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/registratie/infrastructure/brp.adapter.ts

@@ -27,7 +27,12 @@ export function parseBrpAddress(json: unknown): Result<string, BrpAddressDto> {
   if (typeof dto.gevonden !== 'boolean') return err('brp-address: missing/invalid gevonden');
   if (dto.gevonden) {
     const a = dto.adres;
-    if (!a || typeof a.straat !== 'string' || typeof a.postcode !== 'string' || typeof a.woonplaats !== 'string') {
+    if (
+      !a ||
+      typeof a.straat !== 'string' ||
+      typeof a.postcode !== 'string' ||
+      typeof a.woonplaats !== 'string'
+    ) {
       return err('brp-address: missing/invalid adres');
     }
   }