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/value-object/SKILL.md

@@ -28,6 +28,7 @@ export function parsePostcode(raw: string): Result<string, Postcode> {
 ```
 
 Rules:
+
 - The `as Brand` cast appears **only** inside the parser — the type is mintable nowhere else.
 - Error message is user-facing → `$localize` with a stable `@@validation.<name>` id.
 - Trim/normalise before testing; return the cleaned value.