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/new/+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';
 
 export async function load({ locals }: { locals: App.Locals }) {
 	const canWrite =
@@ -26,23 +27,14 @@ export const actions = {
 		const deathYearStr = formData.get('deathYear')?.toString().trim();
 		const notes = formData.get('notes')?.toString().trim() || undefined;
 
-		if (!lastName) {
+		const validationError = validatePersonFields(personType, firstName, lastName);
+		if (validationError) {
 			return fail(400, {
-				error: 'Nachname ist Pflichtfeld.',
+				error: validationError,
 				personType,
 				title,
-				firstName,
-				lastName: '',
-				alias
-			});
-		}
-		if (personType === 'PERSON' && !firstName) {
-			return fail(400, {
-				error: 'Vorname ist Pflichtfeld.',
-				personType,
-				title,
-				firstName: '',
-				lastName,
+				firstName: firstName ?? '',
+				lastName: lastName ?? '',
 				alias
 			});
 		}