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;
+}