feat(persons): add sort toggle to person document list (issue #24)

Extracted sortDocumentsByDate utility with full Vitest coverage (6 tests),
wired it into the person detail page with a DESC/ASC toggle button, and
added an E2E smoke test for the toggle interaction.

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

frontend/e2e/persons.spec.ts

@@ -72,6 +72,25 @@ test.describe('New person', () => {
 	});
 });
 
+test.describe('Person detail — sort toggle', () => {
+	test('sort toggle changes the button label when person has documents', async ({ page }) => {
+		await page.goto('/persons');
+		const firstPerson = page.locator('a[href^="/persons/"]').first();
+		await firstPerson.click();
+		await page.waitForSelector('[data-hydrated]');
+
+		const sortBtn = page.getByRole('button', { name: /Neueste zuerst|Älteste zuerst/i });
+		if (await sortBtn.isVisible()) {
+			await expect(sortBtn).toContainText('Neueste zuerst');
+			await sortBtn.click();
+			await expect(sortBtn).toContainText('Älteste zuerst');
+			await sortBtn.click();
+			await expect(sortBtn).toContainText('Neueste zuerst');
+			await page.screenshot({ path: 'test-results/e2e/person-sort-toggle.png' });
+		}
+	});
+});
+
 test.describe('Conversations', () => {
 	test('shows the empty state when no persons are selected', async ({ page }) => {
 		await page.goto('/conversations');