test(persons): extract validatePersonFields and cover validation branches

- New src/lib/person-validation.ts exports validatePersonFields (pure function)
- 8 unit tests covering: valid PERSON, lastName missing/undefined,
  firstName missing/undefined for PERSON, non-PERSON types without firstName
- Both edit and new-person server actions now call the shared helper instead
  of inline if-chains, making the logic testable and non-duplicated

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/persons/[id]/edit/+page.server.ts

@@ -1,6 +1,7 @@
 import { error, fail, redirect } from '@sveltejs/kit';
 import { createApiClient } from '$lib/api.server';
 import { getErrorMessage } from '$lib/errors';
+import { validatePersonFields } from '$lib/person-validation';
 
 type PersonType = 'PERSON' | 'INSTITUTION' | 'GROUP' | 'UNKNOWN' | 'SKIP';
 
@@ -51,11 +52,9 @@ export const actions = {
 		const birthYear = birthYearStr ? parseInt(birthYearStr, 10) : undefined;
 		const deathYear = deathYearStr ? parseInt(deathYearStr, 10) : undefined;
 
-		if (!lastName) {
-			return fail(400, { updateError: 'Nachname ist Pflichtfeld.' });
-		}
-		if (personType === 'PERSON' && !firstName) {
-			return fail(400, { updateError: 'Vorname ist Pflichtfeld.' });
+		const validationError = validatePersonFields(personType, firstName, lastName);
+		if (validationError) {
+			return fail(400, { updateError: validationError });
 		}
 
 		const api = createApiClient(fetch);