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>
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