feat: add real-time notification system with Socket.IO client

Implements the frontend notification client for TEC-2217:

1. notifications-api.ts — API client for list, unread-count,
   markAsRead, markAllAsRead endpoints
2. notifications-store.ts — Zustand store for notification state
   (recent list, unread count, dropdown open state)
3. use-socket-notifications.ts — Socket.IO hook that connects with
   httpOnly cookie auth, listens for notification:new events,
   auto-reconnects, and syncs unread count on (re)connect
4. notification-bell.tsx — Bell icon with unread badge + dropdown
   showing 10 most recent notifications with time-ago formatting,
   mark-as-read on click, mark-all-as-read, and "Xem tất cả" link
5. notifications-provider.tsx — Provider wired into locale layout
   (inside AuthProvider) to initialize Socket.IO connection
6. Dashboard header — NotificationBell placed before LanguageSwitcher

Added socket.io-client dependency.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/components/providers/notifications-provider.tsx

@@ -0,0 +1,16 @@
+'use client';
+
+import { useSocketNotifications } from '@/lib/hooks/use-socket-notifications';
+
+/**
+ * Provider component that initializes the Socket.IO connection
+ * for real-time notifications. Must be rendered inside AuthProvider.
+ */
+export function NotificationsProvider({
+  children,
+}: {
+  children: React.ReactNode;
+}) {
+  useSocketNotifications();
+  return <>{children}</>;
+}