refactor(notification-bell): use buildCommentHref helper

Drops the inline conditional href construction in favour of the
shared helper. Identical URL shape — behaviour preserved.

Refs #300.

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

frontend/src/lib/components/NotificationBell.svelte

@@ -4,6 +4,7 @@ import { goto } from '$app/navigation';
 import { m } from '$lib/paraglide/messages.js';
 import { clickOutside } from '$lib/actions/clickOutside';
 import { notificationStore } from '$lib/stores/notifications.svelte';
+import { buildCommentHref } from '$lib/utils/commentDeepLink';
 import NotificationDropdown from './NotificationDropdown.svelte';
 
 let open = $state(false);
@@ -31,9 +32,11 @@ function closeDropdown() {
 
 async function handleMarkRead(notification: Parameters<typeof stream.markRead>[0]) {
 	await stream.markRead(notification);
-	const url = notification.annotationId
-		? `/documents/${notification.documentId}?commentId=${notification.referenceId}&annotationId=${notification.annotationId}`
-		: `/documents/${notification.documentId}?commentId=${notification.referenceId}`;
+	const url = buildCommentHref(
+		notification.documentId,
+		notification.referenceId,
+		notification.annotationId
+	);
 	closeDropdown();
 	goto(url);
 }