feat: add unread indicator for completed sessions

[RhysSullivan]

Summary

• Add a hollow circle indicator (○) in the session list for sessions that completed while the user was viewing a different session
• The indicator is automatically cleared when the user navigates to that session

How it works

1. When a session transitions from busy → idle and the user is NOT currently viewing that session, it gets marked as "unread"
2. When the user navigates to a session, it automatically gets marked as "read"
3. In the session list dialog, unread sessions show a ○ indicator in the primary color
4. Sessions that are actively working still show the spinner instead of the unread indicator