fix(notifications): retarget NotificationItem import to singleton store

Left over from the hook→singleton refactor — NotificationDropdown still
imported from the deleted $lib/hooks path.

Part of #285.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/NotificationDropdown.svelte

@@ -1,7 +1,7 @@
 <script lang="ts">
 import { m } from '$lib/paraglide/messages.js';
 import { relativeTime } from '$lib/utils/time';
-import type { NotificationItem } from '$lib/hooks/useNotificationStream.svelte';
+import type { NotificationItem } from '$lib/stores/notifications.svelte';
 
 type Props = {
 	notifications: NotificationItem[];