Problem
The current implementation uses beforeunload with preventDefault() and returnValue, which:

Shows an annoying "Are you sure you want to leave?" dialog to users
Often cancels the AJAX request before it completes (browser kills async requests during unload)
Provides poor UX - users expect to navigate away immediately, not be questioned
Unregisters regardless of user response

Solution
Use navigator.sendBeacon() with beforeunload event (without preventDefault):
Why sendBeacon is better:

No user dialog - Navigation happens immediately without interruption
Guaranteed delivery - Browser queues the request and ensures it completes even after page unload
Built for this use case - Specifically designed for sending analytics/cleanup data during page transitions
Non-blocking - Doesn't delay page unload
More reliable - Survives page termination, unlike regular AJAX requests

Technical details:

Sends a POST request - server endpoint updated accordingly
Works reliably across modern browsers (Chrome, Firefox, Safari, Edge)
Fires only on actual navigation/close, not on tab switching