refactor(person): one author-name fallback, localized

joinNameOrUnknown()/unknownPersonName() in personFormat.ts replace the
three hand-rolled '[Unbekannt]' literals (geschichte/utils,
GeschichtenCard, personOption) — the fallback is now the i18n key
person_unknown (de/en/es), and formatAuthorDisplayName localizes the
server-side literal on the pass-through path.

Review round 3: Felix, Markus, Leonie (7).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/lib/person/personFormat.ts

@@ -1,4 +1,5 @@
 import { formatDate } from '$lib/shared/utils/date';
+import { m } from '$lib/paraglide/messages.js';
 
 type Person = { firstName?: string | null; lastName: string; displayName: string };
 type DocForMeta = {
@@ -17,6 +18,19 @@ function djb2(str: string): number {
 	return Math.abs(hash);
 }
 
+/** Localized fallback when a person has no name parts. */
+export function unknownPersonName(): string {
+	return m.person_unknown();
+}
+
+/**
+ * Single source for the join-names-or-fallback rule. Mirrors the server-side
+ * fallback in GeschichteService.toView (which emits the literal '[Unbekannt]').
+ */
+export function joinNameOrUnknown(firstName?: string | null, lastName?: string | null): string {
+	return [firstName, lastName].filter(Boolean).join(' ').trim() || unknownPersonName();
+}
+
 export function getInitials(name: string): string {
 	const words = name.trim().split(/\s+/).filter(Boolean);
 	if (words.length === 0) return '';

frontend/src/lib/person/personOption.ts

@@ -1,4 +1,5 @@
 import type { components } from '$lib/generated/api';
+import { joinNameOrUnknown } from './personFormat';
 
 type Person = components['schemas']['Person'];
 
@@ -21,6 +22,6 @@ export function toPersonOption(p: {
 }): PersonOption {
 	return {
 		id: p.id,
-		displayName: [p.firstName, p.lastName].filter(Boolean).join(' ') || '[Unbekannt]'
+		displayName: joinNameOrUnknown(p.firstName, p.lastName)
 	};
 }