Session peak indicator on CPU/IO graphs

[DanielRivera1309]

In nmon, there's a ">" marker showing the maximum peak reached during
the session. This is very useful to identify spikes without constantly
watching.

For btop, this could be adapted to its beautiful UI style:

• One of the graph blocks could have a subtle "frame" or border
  marking where the peak was reached during the session
• Or a small vertical line/marker on the graph

This would help sysadmins quickly see if there were spikes they
might have missed, especially useful for:

• Diagnosing intermittent load issues
• Monitoring servers over SSH
• Identifying CPU/memory spikes after the fact

Optional: A toggle in config to enable/disable this feature.

Thank you!