test(activity): assert Chronik rows never fabricate a letter date

Negative guarantee for #668: ChronikRow renders the activity timestamp
(happenedAt), and ActivityFeedItemDTO carries no document-date surface, so
no undated badge or "Datum unbekannt" letter-date label may appear. Pins
this as a regression fixture so a future change can't quietly add a date
chip to the activity feed.

Refs #668

frontend/src/lib/activity/ChronikRow.svelte.spec.ts

@@ -44,6 +44,17 @@ describe('ChronikRow', () => {
 		expect(link).not.toBeNull();
 	});
 
+	// --- #668 negative guarantee: Chronik never fabricates a letter date ---
+	it('renders the activity timestamp, not a letter date, and no undated badge', async () => {
+		// The row shows the relative activity time (happenedAt), never the letter's
+		// documentDate — ActivityFeedItemDTO carries no date surface to badge.
+		render(ChronikRow, { item: baseItem });
+		// No undated badge is introduced into a Chronik row.
+		expect(document.querySelector('[data-testid="undated-badge"]')).toBeNull();
+		// No fabricated "Datum unbekannt" letter-date label appears.
+		await expect.element(page.getByText('Datum unbekannt')).not.toBeInTheDocument();
+	});
+
 	// --- simple variant ---
 	it('renders simple variant when count === 1 and not a mention', async () => {
 		render(ChronikRow, { item: baseItem });