fix(frontend): harden activity split responsiveness

frontend/src/shell/AppShell.tsx

@@ -33,7 +33,7 @@ import type { CSSProperties } from "react";
  * portal's left sidebar, and the workflow phases ride a slim strip above the chat.
  */
 export function AppShell() {
-  const { containerRef, panelWidth, resizing, separatorProps } = useActivityPanelResize();
+  const { containerRef, panelWidth, desktopSplit, resizing, separatorProps } = useActivityPanelResize();
   const activityWidthStyle = {
     "--activity-panel-width": `${panelWidth}px`,
   } as CSSProperties;
@@ -387,9 +387,9 @@ export function AppShell() {
           onResume={doResume}
         />
       )}
-      {showActivity && <ModelActivityPanel onClose={() => setShowActivity(false)} />}
-      {showActivity && (
-        <div className="relative hidden w-0 shrink-0 lg:block">
+      {showActivity && <ModelActivityPanel desktopSplit={desktopSplit} onClose={() => setShowActivity(false)} />}
+      {showActivity && desktopSplit && (
+        <div className="relative w-0 shrink-0">
           <div
             {...separatorProps}
             data-testid="activity-resize-handle"