refactor: move shared utilities to lib/shared/ sub-packages

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

frontend/src/lib/person/personLifeDates.spec.ts

@@ -0,0 +1,24 @@
+import { describe, it, expect } from 'vitest';
+import { formatLifeDateRange } from './personLifeDates';
+
+describe('formatLifeDateRange', () => {
+	it('returns both dates when birth and death year are given', () => {
+		expect(formatLifeDateRange(1882, 1944)).toBe('* 1882 – † 1944');
+	});
+
+	it('returns only birth year when only birthYear is given', () => {
+		expect(formatLifeDateRange(1882, undefined)).toBe('* 1882');
+	});
+
+	it('returns only death year when only deathYear is given', () => {
+		expect(formatLifeDateRange(undefined, 1944)).toBe('† 1944');
+	});
+
+	it('returns empty string when neither year is given', () => {
+		expect(formatLifeDateRange(undefined, undefined)).toBe('');
+	});
+
+	it('returns empty string when both are null', () => {
+		expect(formatLifeDateRange(null, null)).toBe('');
+	});
+});