refactor(date): consolidate formatDate in date.ts with optional format param

Add format?: 'short'|'long' (default 'long') to date.ts formatDate and
remove the duplicate from personFormat.ts. Update DocumentTopBar to
import from date.ts directly. Move the formatDate tests from
personFormat.spec to date.spec.

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

frontend/src/lib/utils/personFormat.ts

@@ -1,4 +1,5 @@
 import { formatDocumentStatus } from './documentStatusLabel';
+import { formatDate } from './date';
 
 type Person = { firstName?: string | null; lastName: string; displayName: string };
 type DocumentStatus = 'PLACEHOLDER' | 'UPLOADED' | 'TRANSCRIBED' | 'REVIEWED' | 'ARCHIVED';
@@ -75,22 +76,6 @@ export function personAvatarColor(personId: string): string {
 	return AVATAR_PALETTE[djb2(personId) % AVATAR_PALETTE.length];
 }
 
-export function formatDate(isoDate: string, format: 'short' | 'long'): string {
-	const date = new Date(isoDate + 'T12:00:00');
-	if (format === 'short') {
-		return new Intl.DateTimeFormat('de-DE', {
-			day: '2-digit',
-			month: '2-digit',
-			year: 'numeric'
-		}).format(date);
-	}
-	return new Intl.DateTimeFormat('de-DE', {
-		day: 'numeric',
-		month: 'long',
-		year: 'numeric'
-	}).format(date);
-}
-
 export function statusDotClass(status: DocumentStatus): string {
 	switch (status) {
 		case 'PLACEHOLDER':