Add session status check button

Add new button to check and display current session status:
- New checkSessionStatus() function in useAuthentik composable
- Displays toast notification with session info
- Shows user name if authenticated or warning if not
- Add UNotifications component to app.vue to render toasts

This allows users to quickly verify their authentication status.

nuxt4/app/app.vue

@@ -1,6 +1,7 @@
 <template>
   <UApp>
     <NuxtRouteAnnouncer />
+    <UNotifications />
 
     <UContainer class="py-8">
       <div class="space-y-6">