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/lib/notifications-api.ts

@@ -0,0 +1,52 @@
+import { apiClient } from './api-client';
+
+// ─── Types ──────────────────────────────────────────────
+
+export interface NotificationDto {
+  id: string;
+  type: string;
+  title: string;
+  body: string;
+  link: string | null;
+  isRead: boolean;
+  createdAt: string;
+}
+
+export interface PaginatedNotifications {
+  data: NotificationDto[];
+  total: number;
+  page: number;
+  limit: number;
+  totalPages: number;
+}
+
+export interface UnreadCountDto {
+  count: number;
+}
+
+// ─── API Functions ──────────────────────────────────────
+
+export const notificationsApi = {
+  /** Get paginated notifications for the current user */
+  list: (params: { page?: number; limit?: number } = {}) => {
+    const query = new URLSearchParams();
+    if (params.page) query.append('page', String(params.page));
+    if (params.limit) query.append('limit', String(params.limit));
+    const qs = query.toString();
+    return apiClient.get<PaginatedNotifications>(
+      `/notifications${qs ? `?${qs}` : ''}`,
+    );
+  },
+
+  /** Get unread notification count */
+  unreadCount: () =>
+    apiClient.get<UnreadCountDto>('/notifications/unread-count'),
+
+  /** Mark a single notification as read */
+  markAsRead: (id: string) =>
+    apiClient.patch<{ success: boolean }>(`/notifications/${id}/read`),
+
+  /** Mark all notifications as read */
+  markAllAsRead: () =>
+    apiClient.patch<{ success: boolean }>('/notifications/read-all'),
+};