refactor(notification): replace callback props with form actions in Dropdown and Bell

NotificationDropdown now wraps each row in a <form action="/aktivitaeten?/dismiss-notification">
and the mark-all control in <form action="/aktivitaeten?/mark-all-read">, wired via use:enhance
for optimistic UI. Props renamed onMarkRead/onMarkAllRead → optimisticMarkRead/optimisticMarkAllRead
to match the simplified store API. NotificationBell passes the store helpers directly; handleMarkRead
is removed.

Test mocks updated: $app/forms enhance mock fires SubmitFunction synchronously on form submit so
callback assertions work without a real HTTP round-trip.

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

frontend/src/lib/notification/NotificationBell.svelte

@@ -1,10 +1,8 @@
 <script lang="ts">
 import { onMount, onDestroy } from 'svelte';
-import { goto } from '$app/navigation';
 import { m } from '$lib/paraglide/messages.js';
 import { clickOutside } from '$lib/shared/actions/clickOutside';
 import { notificationStore } from '$lib/notification/notifications.svelte';
-import { buildCommentHref } from '$lib/shared/discussion/commentDeepLink';
 import NotificationDropdown from './NotificationDropdown.svelte';
 
 let open = $state(false);
@@ -30,17 +28,6 @@ function closeDropdown() {
 	bellButtonEl?.focus();
 }
 
-async function handleMarkRead(notification: Parameters<typeof stream.markRead>[0]) {
-	await stream.markRead(notification);
-	const url = buildCommentHref(
-		notification.documentId,
-		notification.referenceId,
-		notification.annotationId
-	);
-	closeDropdown();
-	goto(url);
-}
-
 function handleKeydown(event: KeyboardEvent) {
 	if (event.key === 'Escape' && open) {
 		event.stopPropagation();
@@ -113,8 +100,8 @@ onDestroy(() => {
 	{#if open}
 		<NotificationDropdown
 			notifications={stream.notifications}
-			onMarkRead={handleMarkRead}
-			onMarkAllRead={stream.markAllRead}
+			optimisticMarkRead={stream.optimisticMarkRead}
+			optimisticMarkAllRead={stream.optimisticMarkAllRead}
 			onClose={closeDropdown}
 		/>
 	{/if}