feat(frontend): rotating activity icon + 5-line expandable activity panel

- CentralStatus: replace the red pulsing dot at the start of the "working" line
  with the rotating WorkingSpinner, now a button that opens the model-activity
  panel (onOpenActivity). WorkingSpinner is extracted to its own module (was
  local to AppShell).
- ModelActivityPanel: show the last 5 lines of the model-stream tail (refreshing
  as the stream grows) with an expand toggle to the full stream, replacing the
  unbounded full transcript.
- AppShell: drop the separate spinner button (the inline icon is now the single
  trigger) and the local WorkingSpinner def.
- Remove the now-orphaned Transcript.tsx (its only consumer was the panel).

TDD: CentralStatus + ModelActivityPanel tests RED->GREEN; frontend suite 87/87;
tsc clean. Live visual verification pending (to do with the B/C frontend pass).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/shell/AppShell.tsx

@@ -176,7 +176,7 @@ export function AppShell() {
           <div className="mx-auto flex max-w-3xl flex-col gap-6">
             {activeSessionId ? (
               <>
-                <CentralStatus working={working} />
+                <CentralStatus working={working} onOpenActivity={toggleActivity} />
                 <WidgetHost sessionId={activeSessionId} />
               </>
             ) : (
@@ -188,20 +188,6 @@ export function AppShell() {
         {/* Sticky composer: the box stays pinned while the conversation scrolls above. */}
         <div className="shrink-0 px-6 pb-6 pt-2">
           <div className="mx-auto max-w-3xl">
-            {activeSessionId && (
-              <div className="mb-1.5 flex justify-start">
-                <button
-                  type="button"
-                  onClick={toggleActivity}
-                  aria-label="Model activity"
-                  aria-pressed={showActivity}
-                  title="Model activity"
-                  className="grid size-7 place-items-center rounded-md border border-border text-primary transition-colors hover:bg-muted"
-                >
-                  <WorkingSpinner spinning={working} />
-                </button>
-              </div>
-            )}
             <div className="rounded-2xl border border-input bg-card shadow-sm transition-colors focus-within:border-primary/40 focus-within:ring-3 focus-within:ring-ring/15">
               <div className="px-2.5 py-2">
                 <SteerInput
@@ -367,22 +353,6 @@ export function AppShell() {
   );
 }
 
-/** Brand-tinted spinner shown while the assistant/harness is working. */
-function WorkingSpinner({ className, spinning = true }: { className?: string; spinning?: boolean }) {
-  return (
-    <svg
-      viewBox="0 0 24 24"
-      fill="none"
-      role="status"
-      aria-label="Assistant is working"
-      className={["size-4 text-primary", spinning ? "animate-spin" : "", className].filter(Boolean).join(" ")}
-    >
-      <circle cx="12" cy="12" r="9" stroke="currentColor" strokeWidth="3" opacity="0.2" />
-      <path d="M21 12a9 9 0 0 0-9-9" stroke="currentColor" strokeWidth="3" strokeLinecap="round" />
-    </svg>
-  );
-}
-
 function EmptyState() {
   return (
     <div className="flex min-h-[60vh] flex-col items-center justify-center text-center">