test(briefwechsel): lock future-date relative-year hiding at the row layer

relativeYearsDe already returns "" for future dates (covered in its
own spec), but the integration wiring inside ThumbnailRow was
untested. Adds a regression that a doc with documentDate in the
future produces no "vor N Jahren" or "vor weniger als 1 Jahr" chip.

Refs #305
Fixes @saraholt concern 5 from PR review

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

frontend/src/lib/components/ThumbnailRow.svelte.spec.ts

@@ -104,6 +104,20 @@ describe('ThumbnailRow', () => {
 		expect(document.body.textContent).toContain('vor 76 Jahren');
 	});
 
+	it('hides the relative-year label when documentDate is in the future', () => {
+		// relativeYearsDe returns "" for future/invalid dates; the row must not
+		// then render an empty chip or print "vor 0 Jahren".
+		render(ThumbnailRow, {
+			doc: { ...baseDoc, documentDate: '2030-01-01' },
+			isOut: true,
+			showOtherParty: false,
+			now: new Date('2026-06-01T00:00:00Z')
+		});
+
+		expect(document.body.textContent).not.toMatch(/vor \d+ Jahr/);
+		expect(document.body.textContent).not.toMatch(/vor weniger/);
+	});
+
 	it('sets border-l class based on isOut', () => {
 		const { unmount } = render(ThumbnailRow, {
 			doc: baseDoc,