fix(app): drop the track panel divider when the list is empty

Bulk archive keeps the panel mounted with zero rows while it is archiving or
after a failure — that is the state it lands in once it has archived every
finished subagent. The divider under the action then had nothing beneath it and
read as a hairline stranded at the bottom of the panel.

packages/app/src/subagents/track.tsx

@@ -83,7 +83,7 @@ export function SubagentsTrack({
       statusBucket={pill.statusBucket}
     >
       {showArchiveFinished && onArchiveFinished ? (
-        <ComposerTrackActions>
+        <ComposerTrackActions divided={rows.length > 0}>
           <ArchiveFinishedRow
             status={archiveFinishedStatus}
             disabled={isArchivingFinished}