test(geschichte): add unit tests for formatAuthorName, formatAuthorDisplayName, formatPublishedAt

13 tests covering null/undefined inputs, partial names, email fallback,
and TZ-safe date slicing for formatPublishedAt.

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

frontend/src/lib/geschichte/utils.test.ts

@@ -0,0 +1,67 @@
+import { describe, it, expect } from 'vitest';
+import { formatAuthorName, formatAuthorDisplayName, formatPublishedAt } from './utils';
+
+describe('formatAuthorName', () => {
+	it('joins firstName and lastName with a space', () => {
+		expect(formatAuthorName({ firstName: 'Anna', lastName: 'Schmidt', email: 'a@x' })).toBe(
+			'Anna Schmidt'
+		);
+	});
+
+	it('returns firstName alone when lastName is absent', () => {
+		expect(formatAuthorName({ firstName: 'Anna', email: 'a@x' })).toBe('Anna');
+	});
+
+	it('returns lastName alone when firstName is absent', () => {
+		expect(formatAuthorName({ lastName: 'Schmidt', email: 'a@x' })).toBe('Schmidt');
+	});
+
+	it('falls back to email when both names are absent', () => {
+		expect(formatAuthorName({ email: 'fallback@example.com' })).toBe('fallback@example.com');
+	});
+
+	it('returns empty string for null input', () => {
+		expect(formatAuthorName(null)).toBe('');
+	});
+
+	it('returns empty string for undefined input', () => {
+		expect(formatAuthorName(undefined)).toBe('');
+	});
+});
+
+describe('formatAuthorDisplayName', () => {
+	it('returns displayName when present', () => {
+		expect(formatAuthorDisplayName({ displayName: 'Anna Schmidt' })).toBe('Anna Schmidt');
+	});
+
+	it('returns empty string for null input', () => {
+		expect(formatAuthorDisplayName(null)).toBe('');
+	});
+
+	it('returns empty string for undefined input', () => {
+		expect(formatAuthorDisplayName(undefined)).toBe('');
+	});
+});
+
+describe('formatPublishedAt', () => {
+	it('returns null for null input', () => {
+		expect(formatPublishedAt(null)).toBeNull();
+	});
+
+	it('returns null for undefined input', () => {
+		expect(formatPublishedAt(undefined)).toBeNull();
+	});
+
+	it('formats an ISO datetime string to a localised date', () => {
+		const result = formatPublishedAt('2026-04-15T10:00:00Z', 'short');
+		expect(result).not.toBeNull();
+		expect(result).toContain('2026');
+	});
+
+	it('slices to date-only before formatting (no TZ off-by-one)', () => {
+		// Both dates should format identically regardless of timezone offset
+		const a = formatPublishedAt('2026-04-15T00:00:00Z', 'short');
+		const b = formatPublishedAt('2026-04-15T23:59:59Z', 'short');
+		expect(a).toBe(b);
+	});
+});