test(activity): migrate ChronikFuerDichBox spec to shared $app/forms mock

Load-bearing first migration (ADR-012): this is the hardest case — its
enhance submit callback actually fires and reads the form result. Replaces
the duplicated 23-line interceptor factory with vi.mock('$app/forms',
() => ({ ...formsMock })) via $mocks, and the per-test mockFormResult
mutation with formsMock.setFormResult({ type: 'failure' }). The reset now
comes from the shared module's embedded beforeEach. The existing
optimisticMarkRead/optimisticMarkAllRead-on-submit assertions remain as the
positive proof the callback fired. Part of #560.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -4,36 +4,12 @@ import { page, userEvent } from 'vitest/browser';
 
 import ChronikFuerDichBox from './ChronikFuerDichBox.svelte';
 import type { NotificationItem } from '$lib/notification/notifications.svelte';
+import * as formsMock from '$mocks/$app/forms';
 
-const mockFormResult = vi.hoisted(() => ({ type: 'success' as string }));
-
-vi.mock('$app/forms', () => ({
-	enhance(
-		node: HTMLFormElement,
-		submit?: (opts: {
-			formData: FormData;
-		}) => (opts: {
-			result: { type: string; data?: Record<string, unknown> };
-			update: () => Promise<void>;
-		}) => Promise<void>
-	) {
-		const handler = async (e: Event) => {
-			e.preventDefault();
-			const cb = submit?.({ formData: new FormData(node) } as never);
-			if (typeof cb === 'function') {
-				await (
-					cb as (o: { result: typeof mockFormResult; update: () => Promise<void> }) => Promise<void>
-				)({ result: mockFormResult, update: async () => {} });
-			}
-		};
-		node.addEventListener('submit', handler);
-		return { destroy: () => node.removeEventListener('submit', handler) };
-	}
-}));
+vi.mock('$app/forms', () => ({ ...formsMock }));
 
 afterEach(() => {
 	cleanup();
-	mockFormResult.type = 'success';
 });
 
 function notif(partial: Partial<NotificationItem>): NotificationItem {
@@ -176,7 +152,7 @@ describe('ChronikFuerDichBox', () => {
 	});
 
 	it('shows an accessible error banner when the dismiss action returns a failure', async () => {
-		mockFormResult.type = 'failure';
+		formsMock.setFormResult({ type: 'failure' });
 		render(ChronikFuerDichBox, {
 			unread: [notif({ id: 'err-1' })],
 			optimisticMarkRead: vi.fn(),