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/composer/tracks.tsx

@@ -156,11 +156,18 @@ function ComposerTrackPillTrigger({
  * means travelling past every row it affects, which is the right price for it, and it cannot sit
  * under a thumb that was aiming for the nearest subagent.
  */
-export function ComposerTrackActions({ children }: { children: ReactNode }): ReactElement {
+export function ComposerTrackActions({
+  children,
+  /** False when the list below is empty — a divider with nothing under it separates nothing. */
+  divided = true,
+}: {
+  children: ReactNode;
+  divided?: boolean;
+}): ReactElement {
   return (
     <>
       {children}
-      <MenuSeparator />
+      {divided ? <MenuSeparator /> : null}
     </>
   );
 }

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}