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/es/common.json

@@ -1097,7 +1097,11 @@
       "default_calendar": "Calendario predeterminado",
       "default_reminder": "Recordatorio predeterminado",
       "time_format_12h": "12 horas",
-      "time_format_24h": "24 horas"
+      "time_format_24h": "24 horas",
+      "notifications_enabled": "Notificaciones de eventos",
+      "notifications_enabled_desc": "Mostrar alertas para eventos próximos",
+      "notification_sound": "Sonido de notificación",
+      "notification_sound_desc": "Reproducir un sonido para las alertas del calendario"
     },
     "days": {
       "monday": "Lunes",
@@ -1121,7 +1125,10 @@
       "event_deleted": "Evento eliminado",
       "calendar_created": "Calendario creado",
       "calendar_deleted": "Calendario eliminado",
-      "event_move_error": "Error al mover el evento"
+      "event_move_error": "Error al mover el evento",
+      "alert_title": "Evento próximo",
+      "alert_now": "Comienza ahora",
+      "alert_in_minutes": "En {count} min"
     },
     "status": {
       "loading_calendars": "Cargando calendarios...",