fix(client): Make `isTimelineAvailable` a reference (#9906)

* Make `isTimelineAvailable` a reference

* Update b.vue

packages/frontend/src/ui/visitor/b.vue

@@ -83,7 +83,7 @@ const announcements = {
 	limit: 10,
 };
 
-const isTimelineAvailable = instance.policies.ltlAvailable || instance.policies.gtlAvailable;
+const isTimelineAvailable = $ref(instance.policies?.ltlAvailable || instance.policies?.gtlAvailable);
 
 let showMenu = $ref(false);
 let isDesktop = $ref(window.innerWidth >= DESKTOP_THRESHOLD);