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.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;
+}