feat(calendar): add event notifications with alert evaluation

Add client-side calendar event notification system that evaluates JMAP
CalendarEventAlert triggers and displays toast notifications when alert
times are reached. Includes configurable notification sound, acknowledged
alert persistence, and proactive event fetching for background alerts.
Also mounts ToastContainer globally to fix silent toast failures.

locales/it/common.json

@@ -1097,7 +1097,11 @@
       "default_calendar": "Calendario predefinito",
       "default_reminder": "Promemoria predefinito",
       "time_format_12h": "12 ore",
-      "time_format_24h": "24 ore"
+      "time_format_24h": "24 ore",
+      "notifications_enabled": "Notifiche eventi",
+      "notifications_enabled_desc": "Mostra avvisi per gli eventi in arrivo",
+      "notification_sound": "Suono di notifica",
+      "notification_sound_desc": "Riproduci un suono per gli avvisi del calendario"
     },
     "days": {
       "monday": "Lunedì",
@@ -1121,7 +1125,10 @@
       "event_deleted": "Evento eliminato",
       "calendar_created": "Calendario creato",
       "calendar_deleted": "Calendario eliminato",
-      "event_move_error": "Spostamento dell'evento non riuscito"
+      "event_move_error": "Spostamento dell'evento non riuscito",
+      "alert_title": "Evento in arrivo",
+      "alert_now": "Inizia ora",
+      "alert_in_minutes": "Tra {count} min"
     },
     "status": {
       "loading_calendars": "Caricamento calendari...",