fix(persons): localize validation error messages via Paraglide i18n

validatePersonFields now returns a PersonValidationKey instead of a
hardcoded German string. resolveValidationMessage() translates the key
through Paraglide so English and Spanish locale users no longer see
German error text. Adds validation_last_name_required and
validation_first_name_required to all three message files.

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

frontend/src/lib/person-validation.ts

@@ -1,3 +1,5 @@
+import { m } from '$lib/paraglide/messages.js';
+
 export const PERSON_TYPES = ['PERSON', 'INSTITUTION', 'GROUP', 'UNKNOWN'] as const;
 export type PersonType = (typeof PERSON_TYPES)[number];
 
@@ -5,12 +7,22 @@ export function normalizePersonType(raw: string | undefined | null): PersonType
 	return raw === 'SKIP' ? 'UNKNOWN' : ((raw ?? 'PERSON') as PersonType);
 }
 
+export type PersonValidationKey =
+	| 'validation_last_name_required'
+	| 'validation_first_name_required';
+
+export function resolveValidationMessage(key: PersonValidationKey): string {
+	return key === 'validation_last_name_required'
+		? m.validation_last_name_required()
+		: m.validation_first_name_required();
+}
+
 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.';
+): PersonValidationKey | null {
+	if (!lastName) return 'validation_last_name_required';
+	if (personType === 'PERSON' && !firstName) return 'validation_first_name_required';
 	return null;
 }