test(activity): cover ChronikErrorCard and ChronikTimeline

ChronikErrorCard: default vs supplied message, retry callback wiring,
role=alert.

ChronikTimeline: empty render, today bucket grouping, older bucket
grouping, multi-bucket grouping when items span time ranges.

8 tests, ~20 branches.

Refs #496.

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

frontend/src/lib/activity/ChronikErrorCard.svelte.test.ts

@@ -0,0 +1,37 @@
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+import ChronikErrorCard from './ChronikErrorCard.svelte';
+
+afterEach(cleanup);
+
+describe('ChronikErrorCard', () => {
+	it('renders the default error message when no message is supplied', async () => {
+		render(ChronikErrorCard, { props: { onRetry: () => {} } });
+
+		await expect.element(page.getByText(/Aktivitäten konnten nicht/i)).toBeVisible();
+	});
+
+	it('renders the supplied message when provided', async () => {
+		render(ChronikErrorCard, {
+			props: { onRetry: () => {}, message: 'Custom error message' }
+		});
+
+		await expect.element(page.getByText('Custom error message')).toBeVisible();
+	});
+
+	it('calls onRetry when the retry button is clicked', async () => {
+		const onRetry = vi.fn();
+		render(ChronikErrorCard, { props: { onRetry } });
+
+		await page.getByRole('button', { name: /erneut versuchen/i }).click();
+
+		expect(onRetry).toHaveBeenCalledOnce();
+	});
+
+	it('marks the card as role="alert" for assistive tech', async () => {
+		render(ChronikErrorCard, { props: { onRetry: () => {} } });
+
+		await expect.element(page.getByRole('alert')).toBeVisible();
+	});
+});