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/date.spec.ts

@@ -1,5 +1,25 @@
 import { describe, expect, it } from 'vitest';
-import { formatGermanDateInput, isoToGerman, germanToIso } from './date';
+import { formatDate, formatGermanDateInput, isoToGerman, germanToIso } from './date';
+
+// ─── formatDate ──────────────────────────────────────────────────────────────
+
+describe('formatDate', () => {
+	it('defaults to long format when no format arg is passed', () => {
+		expect(formatDate('1943-12-24')).toBe('24. Dezember 1943');
+	});
+
+	it('formats long date with German month name', () => {
+		expect(formatDate('1943-12-24', 'long')).toBe('24. Dezember 1943');
+	});
+
+	it('formats short date as dd.mm.yyyy', () => {
+		expect(formatDate('1943-12-24', 'short')).toBe('24.12.1943');
+	});
+
+	it('does not shift Dec 31 to Jan 1 (T12:00:00 UTC guard)', () => {
+		expect(formatDate('1943-12-31', 'short')).toBe('31.12.1943');
+	});
+});
 
 // ─── isoToGerman ─────────────────────────────────────────────────────────────