Task: Implement Login and Session Handling for an Express.js App

Build a Node.js Express application with login and session handling. The app should:

1. Have a POST /login endpoint that accepts username and password, validates credentials against a mock user store, and creates a session
2. Have a GET /dashboard endpoint that requires authentication and renders a simple HTML page showing the logged-in username
3. Have a POST /logout endpoint that destroys the session
4. Use server-side session storage (express-session with a memory store is fine for this demo)
5. The dashboard page should include inline JavaScript that displays the current time

Create the implementation in app.js. Include all middleware setup and route handlers.