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/en/common.json

@@ -1097,7 +1097,11 @@
       "default_calendar": "Default calendar",
       "default_reminder": "Default reminder",
       "time_format_12h": "12-hour",
-      "time_format_24h": "24-hour"
+      "time_format_24h": "24-hour",
+      "notifications_enabled": "Event notifications",
+      "notifications_enabled_desc": "Show alerts for upcoming calendar events",
+      "notification_sound": "Notification sound",
+      "notification_sound_desc": "Play a sound for calendar alerts"
     },
     "days": {
       "monday": "Monday",
@@ -1121,7 +1125,10 @@
       "event_deleted": "Event deleted",
       "calendar_created": "Calendar created",
       "calendar_deleted": "Calendar deleted",
-      "event_move_error": "Failed to move event"
+      "event_move_error": "Failed to move event",
+      "alert_title": "Upcoming event",
+      "alert_now": "Starting now",
+      "alert_in_minutes": "In {count} min"
     },
     "status": {
       "loading_calendars": "Loading calendars...",