Real-time Notification Stream

Problem Description

A project management tool needs a real-time notification system. Users receive notifications when they are assigned to a task, when a task they are watching is updated, or when someone mentions them in a comment. Notifications should appear instantly without refreshing the page.

Build a Node.js/Express backend and React/TypeScript frontend:

• Backend has endpoints to create notifications for specific users
• Notifications are pushed to the targeted user in real time
• The UI shows an unread notification count badge and a dropdown list of recent notifications
• Users can mark notifications as read
• Notifications have: id, userId, type (assignment | update | mention), title, body, read, createdAt
• Only the targeted user should receive their notifications (not broadcast to everyone)

Output Specification

Produce the following in a src/ directory:

Server:

• src/server.ts -- Express server with real-time notification delivery
• src/notifications.ts -- Notification storage and delivery logic

Client:

• src/components/NotificationBell.tsx -- The notification icon with unread count badge
• src/components/NotificationList.tsx -- Dropdown list of notifications
• src/hooks/useNotifications.ts -- Hook that manages the real-time notification connection

Use in-memory storage. Do not include test files or build configuration.