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/lib/person-validation.test.ts

@@ -0,0 +1,38 @@
+import { describe, it, expect } from 'vitest';
+import { validatePersonFields } from './person-validation';
+
+describe('validatePersonFields', () => {
+	it('returns null when all required fields are present for PERSON', () => {
+		expect(validatePersonFields('PERSON', 'Hans', 'Müller')).toBeNull();
+	});
+
+	it('returns lastName error when lastName is missing', () => {
+		expect(validatePersonFields('PERSON', 'Hans', '')).toBe('Nachname ist Pflichtfeld.');
+	});
+
+	it('returns lastName error when lastName is undefined', () => {
+		expect(validatePersonFields('INSTITUTION', undefined, undefined)).toBe(
+			'Nachname ist Pflichtfeld.'
+		);
+	});
+
+	it('returns firstName error when type is PERSON and firstName is missing', () => {
+		expect(validatePersonFields('PERSON', '', 'Müller')).toBe('Vorname ist Pflichtfeld.');
+	});
+
+	it('returns firstName error when type is PERSON and firstName is undefined', () => {
+		expect(validatePersonFields('PERSON', undefined, 'Müller')).toBe('Vorname ist Pflichtfeld.');
+	});
+
+	it('returns null for INSTITUTION without firstName', () => {
+		expect(validatePersonFields('INSTITUTION', undefined, 'Verlag GmbH')).toBeNull();
+	});
+
+	it('returns null for GROUP without firstName', () => {
+		expect(validatePersonFields('GROUP', undefined, 'Familie Raddatz')).toBeNull();
+	});
+
+	it('returns null for UNKNOWN without firstName', () => {
+		expect(validatePersonFields('UNKNOWN', undefined, 'Unbekannt')).toBeNull();
+	});
+});

frontend/src/lib/person-validation.ts

@@ -0,0 +1,9 @@
+export function validatePersonFields(
+	personType: string,
+	firstName: string | undefined | null,
+	lastName: string | undefined | null
+): string | null {
+	if (!lastName) return 'Nachname ist Pflichtfeld.';
+	if (personType === 'PERSON' && !firstName) return 'Vorname ist Pflichtfeld.';
+	return null;
+}

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);

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
 			});
 		}