test(documents): add regression test for sort fallback to year grouping

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

frontend/src/routes/DocumentList.svelte.spec.ts

@@ -104,6 +104,20 @@ describe('DocumentList – year grouping', () => {
 	});
 });
 
+// ─── Sort fallback ────────────────────────────────────────────────────────────
+
+describe('DocumentList – sort fallback', () => {
+	it('falls back to year grouping when sort is not SENDER or RECEIVER', async () => {
+		const items = [
+			makeItem({ document: { ...makeItem().document, id: '1', documentDate: '2024-03-15' } })
+		];
+		render(DocumentList, { ...baseProps, items, total: 1, sort: 'TITLE' });
+		await expect
+			.element(page.getByTestId('group-header').filter({ hasText: '2024' }))
+			.toBeInTheDocument();
+	});
+});
+
 // ─── Sender grouping ─────────────────────────────────────────────────────────
 
 describe('DocumentList – sender grouping', () => {