[Feature]: Mark a session as unread from the sidebar

Problem / Use Case

Once a session has been opened it is marked as read, and there is no way to flag it again as something to come back to. I often open a session to glance at it, then want to keep it highlighted as "still needs my attention", like in a mail or chat client.

Proposed Solution

Add a "Mark as unread" action to the session context menu in the sidebar (next to "Rename task" and "Manage permissions"), plus a keyboard shortcut for the selected session. The session then shows the same unread indicator it has before being opened for the first time.

Additional Context

Related Issues/PRs:

• #1110 (hide tasks from the sidebar, same context menu)