Add Session::last_focused_at, shipped on the wire like every other
field. Both POST /api/sessions/:id/focus (focused: true) and opening
/api/attach/:id stamp it and emit SessionUpdated, so a client that
only attaches sorts the same as one that reports focus explicitly.
Losing focus does not clear it, and a never-focused session has None,
which is what lets the command bar's empty-query ordering
(docs/COMMAND-BAR.md) sort it after every session that has focused.
Miguel Palhas
9d9aef119d