fix(frontend): harden activity split responsiveness

frontend/src/shell/ModelActivityPanel.tsx

@@ -37,7 +37,7 @@ export function formatModelActivity(text: string): string {
 function MarkdownBody({ entry }: { entry: ActivityEntry }) {
   const tone = entry.kind === "thinking" ? "text-muted-foreground" : "text-foreground";
   return (
-    <div className={`thot-prose max-w-[74ch] text-sm ${tone}`}>
+    <div className={`thot-prose thot-activity-prose max-w-[74ch] text-sm ${tone}`}>
       <ReactMarkdown remarkPlugins={[remarkGfm]}>{formatModelActivity(entry.text)}</ReactMarkdown>
     </div>
   );
@@ -56,7 +56,13 @@ function ActivityRow({ entry }: { entry: ActivityEntry }) {
 }
 
 /** Left drawer showing the user-facing projection of the in-memory model activity timeline. */
-export function ModelActivityPanel({ onClose }: { onClose: () => void }) {
+export function ModelActivityPanel({
+  desktopSplit = false,
+  onClose,
+}: {
+  desktopSplit?: boolean;
+  onClose: () => void;
+}) {
   const activityLog = useSessionStore((s) => s.activityLog);
   const visibleActivity = activityLog.filter(isVisibleModelActivity);
   const visibleCount = visibleActivity.length;
@@ -70,7 +76,10 @@ export function ModelActivityPanel({ onClose }: { onClose: () => void }) {
   }, [visibleCount, visibleTail]);
 
   return (
-    <aside className="fixed inset-y-0 left-0 z-30 flex w-[min(90vw,24rem)] shrink-0 flex-col border-r border-border bg-sidebar lg:static lg:z-auto lg:w-[var(--activity-panel-width)] lg:border-r-0">
+    <aside className={desktopSplit
+      ? "static z-auto flex w-[var(--activity-panel-width)] shrink-0 flex-col border-r-0 bg-sidebar"
+      : "fixed inset-y-0 left-0 z-30 flex w-[min(90vw,24rem)] shrink-0 flex-col border-r border-border bg-sidebar"
+    }>
       <div className="flex items-center justify-between border-b border-border/60 px-3 py-2">
         <h2 className="font-heading text-sm font-semibold text-foreground">Model activity</h2>
         <Button size="icon-sm" variant="ghost" aria-label="Close model activity" onClick={onClose}>