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.

app/[locale]/layout.tsx

@@ -1,6 +1,7 @@
 import { notFound } from "next/navigation";
 import { IntlProvider } from "@/components/providers/intl-provider";
 import { ThemeProvider } from "@/components/providers/theme-provider";
+import { CalendarAlertProvider } from "@/components/providers/calendar-alert-provider";
 import { locales } from "@/i18n/routing";
 
 export default async function LocaleLayout({
@@ -24,7 +25,9 @@ export default async function LocaleLayout({
   return (
     <IntlProvider locale={locale} messages={messages}>
       <ThemeProvider>
-        {children}
+        <CalendarAlertProvider>
+          {children}
+        </CalendarAlertProvider>
       </ThemeProvider>
     </IntlProvider>
   );

app/[locale]/page.tsx

@@ -18,6 +18,7 @@ import { useUIStore } from "@/stores/ui-store";
 import { useDeviceDetection } from "@/hooks/use-media-query";
 import { useKeyboardShortcuts } from "@/hooks/use-keyboard-shortcuts";
 import { debug } from "@/lib/debug";
+import { playNotificationSound } from "@/lib/notification-sound";
 import { cn } from "@/lib/utils";
 import {
   ErrorBoundary,
@@ -87,28 +88,6 @@ export default function Home() {
     advancedSearch,
   } = useEmailStore();
 
-  // Play notification sound for new emails
-  const playNotificationSound = () => {
-    try {
-      // Use Web Audio API for a simple notification beep
-      const audioContext = new (window.AudioContext || (window as unknown as { webkitAudioContext: typeof AudioContext }).webkitAudioContext)();
-      const oscillator = audioContext.createOscillator();
-      const gainNode = audioContext.createGain();
-
-      oscillator.connect(gainNode);
-      gainNode.connect(audioContext.destination);
-
-      oscillator.frequency.value = 800; // Hz
-      oscillator.type = 'sine';
-      gainNode.gain.value = 0.1; // Low volume
-
-      oscillator.start();
-      oscillator.stop(audioContext.currentTime + 0.15); // Short beep
-    } catch (e) {
-      debug.log('Could not play notification sound:', e);
-    }
-  };
-
   // Keyboard shortcuts handlers
   const keyboardHandlers = useMemo(() => ({
     onNextEmail: () => {