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/WorkingSpinner.tsx

@@ -0,0 +1,24 @@
+/** The rotating "assistant is working" icon. Spins while `spinning`; used as the
+ *  inline activity trigger in CentralStatus. */
+export 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>
+  );
+}