refactor(notification): rename markRead/markAllRead to optimistic helpers without fetch

Removes raw fetch() calls from the store. optimisticMarkRead(id) and
optimisticMarkAllRead() now only mutate local $state — the actual API
calls move to SvelteKit form actions on /aktivitaeten.

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

frontend/src/lib/notification/notifications.svelte.ts

@@ -35,28 +35,19 @@ async function fetchUnreadCount(): Promise<void> {
 	}
 }
 
-async function markRead(notification: NotificationItem): Promise<void> {
-	if (!notification.read) {
-		try {
-			await fetch(`/api/notifications/${notification.id}/read`, { method: 'PATCH' });
-			notification.read = true;
-			unreadCount = Math.max(0, unreadCount - 1);
-		} catch (e) {
-			console.error('Failed to mark notification as read', e);
-		}
+function optimisticMarkRead(id: string): void {
+	const notification = notifications.find((n) => n.id === id);
+	if (notification && !notification.read) {
+		notification.read = true;
+		unreadCount = Math.max(0, unreadCount - 1);
 	}
 }
 
-async function markAllRead(): Promise<void> {
-	try {
-		await fetch('/api/notifications/read-all', { method: 'POST' });
-		for (const n of notifications) {
-			n.read = true;
-		}
-		unreadCount = 0;
-	} catch (e) {
-		console.error('Failed to mark all notifications as read', e);
+function optimisticMarkAllRead(): void {
+	for (const n of notifications) {
+		n.read = true;
 	}
+	unreadCount = 0;
 }
 
 function init(): void {
@@ -123,8 +114,8 @@ export const notificationStore = {
 	},
 	fetchNotifications,
 	fetchUnreadCount,
-	markRead,
-	markAllRead,
+	optimisticMarkRead,
+	optimisticMarkAllRead,
 	init,
 	destroy
 };