fix(chronik): surface action failures in ChronikFuerDichBox with accessible error banner

Add $state errorMessage + role=alert banner to ChronikFuerDichBox. Both enhance callbacks
now inspect result.type and set the error message on 'failure' or 'error'; errorMessage
is cleared on each new submit attempt.

Upgrade both test files to the mockFormResult pattern (via vi.hoisted) so the result
callback is exercised. Add a failing-action test in each file that asserts role=alert
appears after a form submit with type='failure'.

Fix bare Function cast → explicit typed cast to satisfy @typescript-eslint/no-unsafe-function-type.

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

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

@@ -5,18 +5,36 @@ import { page, userEvent } from 'vitest/browser';
 import ChronikFuerDichBox from './ChronikFuerDichBox.svelte';
 import type { NotificationItem } from '$lib/notification/notifications.svelte';
 
+const mockFormResult = vi.hoisted(() => ({ type: 'success' as string }));
+
 vi.mock('$app/forms', () => ({
-	enhance(node: HTMLFormElement, submit?: (opts: { formData: FormData }) => unknown) {
-		const handler = (e: Event) => {
+	enhance(
+		node: HTMLFormElement,
+		submit?: (opts: {
+			formData: FormData;
+		}) => (opts: {
+			result: { type: string; data?: Record<string, unknown> };
+			update: () => Promise<void>;
+		}) => Promise<void>
+	) {
+		const handler = async (e: Event) => {
 			e.preventDefault();
-			submit?.({ formData: new FormData(node) } as never);
+			const cb = submit?.({ formData: new FormData(node) } as never);
+			if (typeof cb === 'function') {
+				await (
+					cb as (o: { result: typeof mockFormResult; update: () => Promise<void> }) => Promise<void>
+				)({ result: mockFormResult, update: async () => {} });
+			}
 		};
 		node.addEventListener('submit', handler);
 		return { destroy: () => node.removeEventListener('submit', handler) };
 	}
 }));
 
-afterEach(cleanup);
+afterEach(() => {
+	cleanup();
+	mockFormResult.type = 'success';
+});
 
 function notif(partial: Partial<NotificationItem>): NotificationItem {
 	return {
@@ -156,4 +174,22 @@ describe('ChronikFuerDichBox', () => {
 		// Prevents the senior-audience tap-drag bug flagged by Leonie.
 		expect(dismiss?.closest('a')).toBeNull();
 	});
+
+	it('shows an accessible error banner when the dismiss action returns a failure', async () => {
+		mockFormResult.type = 'failure';
+		render(ChronikFuerDichBox, {
+			unread: [notif({ id: 'err-1' })],
+			optimisticMarkRead: vi.fn(),
+			optimisticMarkAllRead: vi.fn()
+		});
+		const dismiss = document.querySelector(
+			'[data-testid="chronik-fuerdich-dismiss"]'
+		) as HTMLButtonElement | null;
+		expect(dismiss).not.toBeNull();
+		dismiss?.click();
+		// Allow microtask queue to flush
+		await new Promise((r) => setTimeout(r, 0));
+		const alert = document.querySelector('[role="alert"]');
+		expect(alert).not.toBeNull();
+	});
 });