✨(frontend) implement persistent notifications for waiting participants

Add special non-closing notifications for waiting participants that remain
visible until all have been reviewed. Implement complementary system alongside
existing toaster notifications.

Designed with accessibility in mind but would benefit from expert review.
Current UX provides good foundation with quick actions planned for v2.

src/frontend/src/locales/en/notifications.json

@@ -15,5 +15,10 @@
   },
   "reaction": {
     "description": "{{name}} reacted with {{emoji}}"
+  },
+  "waitingParticipants": {
+    "one": "One person wants to join this call.",
+    "several": "Several people want to join this call.",
+    "open": "Open"
   }
 }