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/de/common.json

@@ -1097,7 +1097,11 @@
       "default_calendar": "Standardkalender",
       "default_reminder": "Standarderinnerung",
       "time_format_12h": "12-Stunden",
-      "time_format_24h": "24-Stunden"
+      "time_format_24h": "24-Stunden",
+      "notifications_enabled": "Ereignisbenachrichtigungen",
+      "notifications_enabled_desc": "Benachrichtigungen für bevorstehende Termine anzeigen",
+      "notification_sound": "Benachrichtigungston",
+      "notification_sound_desc": "Ton für Kalenderbenachrichtigungen abspielen"
     },
     "days": {
       "monday": "Montag",
@@ -1121,7 +1125,10 @@
       "event_deleted": "Termin gelöscht",
       "calendar_created": "Kalender erstellt",
       "calendar_deleted": "Kalender gelöscht",
-      "event_move_error": "Termin konnte nicht verschoben werden"
+      "event_move_error": "Termin konnte nicht verschoben werden",
+      "alert_title": "Bevorstehender Termin",
+      "alert_now": "Beginnt jetzt",
+      "alert_in_minutes": "In {count} Min."
     },
     "status": {
       "loading_calendars": "Kalender werden geladen...",