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/calendar-notification-store.ts

@@ -0,0 +1,49 @@
+import { create } from 'zustand';
+import { persist } from 'zustand/middleware';
+
+const RETENTION_THRESHOLD_MS = 24 * 60 * 60 * 1000;
+
+interface CalendarNotificationStore {
+  acknowledgedAlerts: Record<string, number>;
+  acknowledgeAlert: (key: string, fireTimeMs: number) => void;
+  isAcknowledged: (key: string) => boolean;
+  cleanupStaleAlerts: () => void;
+  clearAll: () => void;
+}
+
+export const useCalendarNotificationStore = create<CalendarNotificationStore>()(
+  persist(
+    (set, get) => ({
+      acknowledgedAlerts: {},
+
+      acknowledgeAlert: (key, fireTimeMs) => {
+        set((state) => ({
+          acknowledgedAlerts: { ...state.acknowledgedAlerts, [key]: fireTimeMs },
+        }));
+      },
+
+      isAcknowledged: (key) => {
+        return key in get().acknowledgedAlerts;
+      },
+
+      cleanupStaleAlerts: () => {
+        const now = Date.now();
+        const cleaned = Object.fromEntries(
+          Object.entries(get().acknowledgedAlerts)
+            .filter(([, fireTimeMs]) => now - fireTimeMs < RETENTION_THRESHOLD_MS)
+        );
+        set({ acknowledgedAlerts: cleaned });
+      },
+
+      clearAll: () => {
+        set({ acknowledgedAlerts: {} });
+      },
+    }),
+    {
+      name: 'calendar-notification-storage',
+      partialize: (state) => ({
+        acknowledgedAlerts: state.acknowledgedAlerts,
+      }),
+    }
+  )
+);