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.

stores/settings-store.ts

@@ -37,6 +37,10 @@ interface SettingsState {
   sessionTimeout: number; // minutes (0 = never)
   trustedSenders: string[]; // Email addresses that can load external content
 
+  // Calendar Notifications
+  calendarNotificationsEnabled: boolean;
+  calendarNotificationSound: boolean;
+
   // Advanced
   debugMode: boolean;
 
@@ -82,6 +86,10 @@ const DEFAULT_SETTINGS = {
   sessionTimeout: 0, // Never
   trustedSenders: [] as string[],
 
+  // Calendar Notifications
+  calendarNotificationsEnabled: true,
+  calendarNotificationSound: true,
+
   // Advanced
   debugMode: false,
 };
@@ -136,6 +144,8 @@ export const useSettingsStore = create<SettingsState>()(
           sendConfirmation: state.sendConfirmation,
           defaultReplyMode: state.defaultReplyMode,
           sessionTimeout: state.sessionTimeout,
+          calendarNotificationsEnabled: state.calendarNotificationsEnabled,
+          calendarNotificationSound: state.calendarNotificationSound,
           debugMode: state.debugMode,
         };
         return JSON.stringify(settings, null, 2);