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/geschichte/GeschichtenCard.svelte

@@ -3,6 +3,7 @@ import { m } from '$lib/paraglide/messages.js';
 import type { components } from '$lib/generated/api';
 import { plainExcerpt } from '$lib/shared/utils/extractText';
 import { formatDate } from '$lib/shared/utils/date';
+import { formatAuthorName } from './utils';
 
 type GeschichteSummary = components['schemas']['GeschichteSummary'];
 
@@ -24,10 +25,7 @@ function formatPublishedDate(g: GeschichteSummary): string | null {
 }
 
 function authorName(g: GeschichteSummary): string {
-	const a = g.author;
-	if (!a) return '';
-	const full = [a.firstName, a.lastName].filter(Boolean).join(' ').trim();
-	return full || '[Unbekannt]';
+	return formatAuthorName(g.author);
 }
 </script>