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/fr/common.json

@@ -1097,7 +1097,11 @@
       "default_calendar": "Calendrier par défaut",
       "default_reminder": "Rappel par défaut",
       "time_format_12h": "12 heures",
-      "time_format_24h": "24 heures"
+      "time_format_24h": "24 heures",
+      "notifications_enabled": "Notifications d'événements",
+      "notifications_enabled_desc": "Afficher les alertes pour les événements à venir",
+      "notification_sound": "Son de notification",
+      "notification_sound_desc": "Jouer un son pour les alertes de calendrier"
     },
     "days": {
       "monday": "Lundi",
@@ -1121,7 +1125,10 @@
       "event_deleted": "Événement supprimé",
       "calendar_created": "Calendrier créé",
       "calendar_deleted": "Calendrier supprimé",
-      "event_move_error": "Échec du déplacement de l'événement"
+      "event_move_error": "Échec du déplacement de l'événement",
+      "alert_title": "Événement à venir",
+      "alert_now": "Commence maintenant",
+      "alert_in_minutes": "Dans {count} min"
     },
     "status": {
       "loading_calendars": "Chargement des calendriers...",