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/ja/common.json

@@ -1097,7 +1097,11 @@
       "default_calendar": "デフォルトのカレンダー",
       "default_reminder": "デフォルトのリマインダー",
       "time_format_12h": "12時間制",
-      "time_format_24h": "24時間制"
+      "time_format_24h": "24時間制",
+      "notifications_enabled": "イベント通知",
+      "notifications_enabled_desc": "予定のイベントのアラートを表示する",
+      "notification_sound": "通知音",
+      "notification_sound_desc": "カレンダーアラートの音を鳴らす"
     },
     "days": {
       "monday": "月曜日",
@@ -1121,7 +1125,10 @@
       "event_deleted": "予定を削除しました",
       "calendar_created": "カレンダーを作成しました",
       "calendar_deleted": "カレンダーを削除しました",
-      "event_move_error": "イベントの移動に失敗しました"
+      "event_move_error": "イベントの移動に失敗しました",
+      "alert_title": "予定のイベント",
+      "alert_now": "まもなく開始",
+      "alert_in_minutes": "{count}分後"
     },
     "status": {
       "loading_calendars": "カレンダーを読み込み中...",