feat(relativeTime): add relativeYearsDe helper for historical letter dates

The correspondence timeline labels each row with its distance from today
("vor 86 Jahren"). Uses calendar-field math so the anniversary day
flips exactly — an ms-based 365.25d average misses by a day on leap
years. Invalid / future dates return "" so the caller can hide the
label rather than print "vor 0 Jahren".

Refs #305

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/relativeTime.spec.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest';
-import { relativeTimeDe } from './relativeTime';
+import { relativeTimeDe, relativeYearsDe } from './relativeTime';
 
 const NOW = new Date('2026-04-20T12:00:00Z');
 
@@ -39,3 +39,31 @@ describe('relativeTimeDe', () => {
 		expect(relativeTimeDe(invalid, NOW)).toMatch(/Minute/i);
 	});
 });
+
+describe('relativeYearsDe', () => {
+	it('returns singular "vor 1 Jahr" for exactly one whole year ago', () => {
+		const from = new Date('2025-04-20T12:00:00Z');
+		expect(relativeYearsDe(from, NOW)).toBe('vor 1 Jahr');
+	});
+
+	it('returns plural "vor N Jahren" for more than one year', () => {
+		const from = new Date('1940-04-20T12:00:00Z');
+		expect(relativeYearsDe(from, NOW)).toBe('vor 86 Jahren');
+	});
+
+	it('floors a partial year down (eleven months ago = 0 years)', () => {
+		const from = new Date('2025-06-01T00:00:00Z');
+		// We show "vor weniger als 1 Jahr" rather than rounding up to 1.
+		expect(relativeYearsDe(from, NOW)).toBe('vor weniger als 1 Jahr');
+	});
+
+	it('returns empty string when the input Date is invalid', () => {
+		const invalid = new Date('not-a-real-date');
+		expect(relativeYearsDe(invalid, NOW)).toBe('');
+	});
+
+	it('returns empty string for future dates', () => {
+		const future = new Date('2030-01-01T00:00:00Z');
+		expect(relativeYearsDe(future, NOW)).toBe('');
+	});
+});