Implement inactivity warning notification for imminent session expiration

As a user, 
I want to receive a notification when my session is about 
to expire due to inactivity 
so that I can extend my session and avoid losing any unsaved data or progress.

Background

Currently, sessions expire after 20 minutes of inactivity without warning, leading users to lose their progress unexpectedly. Implementing an inactivity warning will improve user experience by alerting them shortly before their session expires, giving them an option to extend it.

Acceptance Criteria

• A UI-based notification appears 2 minutes before the session is set to expire, informing the user of the impending session expiration due to inactivity.
• Notification clearly states:
  • The remaining time until expiration.
  • The option to extend the session to prevent automatic logout.
• If no action is taken, the session expires as expected, leading to the existing "Session Expired" page.

#42 (comment 741985)

https://design-system.dwp.gov.uk/patterns/manage-a-session-timeout/examples/default#