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/mutation-command/SKILL.md

@@ -17,7 +17,9 @@ machine's `Valid` type, returns the server's answer, no parse (server is authori
 @Injectable({ providedIn: 'root' })
 export class ChangeRequestAdapter {
   private client = inject(ApiClient);
-  async changeRequest(data: Valid): Promise<string> { /* → server reference */ }
+  async changeRequest(data: Valid): Promise<string> {
+    /* → server reference */
+  }
 }
 ```