refactor(chronik): replace callback props with form actions in ChronikFuerDichBox

Dismiss (X) button and mark-all-read button now submit forms to
/aktivitaeten?/dismiss-notification and /aktivitaeten?/mark-all-read respectively.
Props renamed onMarkRead/onMarkAllRead → optimisticMarkRead/optimisticMarkAllRead.

aktivitaeten/+page.svelte drops the now-deleted onMarkRead/onMarkAllRead wrapper functions
and passes notificationStore.optimisticMarkRead/optimisticMarkAllRead directly to the box.

Tests: $app/forms enhance mock added to both spec files so dismiss and mark-all assertions
work synchronously against form-submit events.

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

frontend/src/routes/aktivitaeten/+page.svelte

@@ -76,14 +76,6 @@ async function onFilterChange(v: FilterValue) {
 	});
 }
 
-async function onMarkRead(n: NotificationItem) {
-	await notificationStore.markRead(n);
-}
-
-async function onMarkAllRead() {
-	await notificationStore.markAllRead();
-}
-
 const displayFeed = $derived(applyClientFilter(data.activityFeed, data.filter));
 
 const isEmpty = $derived(displayFeed.length === 0);
@@ -108,7 +100,11 @@ function retry() {
 	{#if data.loadError === 'activity'}
 		<ChronikErrorCard onRetry={retry} />
 	{:else}
-		<ChronikFuerDichBox unread={unread} onMarkRead={onMarkRead} onMarkAllRead={onMarkAllRead} />
+		<ChronikFuerDichBox
+			unread={unread}
+			optimisticMarkRead={notificationStore.optimisticMarkRead}
+			optimisticMarkAllRead={notificationStore.optimisticMarkAllRead}
+		/>
 
 		<div class="mt-6">
 			<ChronikFilterPills value={data.filter} onChange={onFilterChange} />