feat(frontend): central 'working' indicator — elapsed time + model-stream tail

While the model works (active session, no pending widget), CentralStatus shows
a Claude-Code-style live status: a pulsing dot, elapsed seconds (ticking), and a
short tail of the model's current output — so the centre no longer looks stuck
during the long F1 loop. Verbose stream stays in the left panel. No tokens yet
(would need the SessionBridge to forward Pi usage).

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

frontend/src/shell/CentralStatus.test.tsx

@@ -5,8 +5,8 @@ import { CentralStatus } from "./CentralStatus";
 
 beforeEach(() => useSessionStore.getState().resetSession());
 
-test("renders nothing when there is no user entry and no step messages", () => {
-  const { container } = render(<CentralStatus />);
+test("renders nothing when not working and there is no user entry or step messages", () => {
+  const { container } = render(<CentralStatus working={false} />);
   expect(container).toBeEmptyDOMElement();
 });
 
@@ -14,7 +14,22 @@ test("echoes the last user entry and the step messages", () => {
   const st = useSessionStore.getState();
   st.setLastUserEntry({ kind: "input", text: "how many patients?" });
   st.applyEvent({ type: "info", level: "info", text: "Searching the schema…" });
-  render(<CentralStatus />);
+  render(<CentralStatus working={false} />);
   expect(screen.getByText("how many patients?")).toBeInTheDocument();
   expect(screen.getByText("Searching the schema…")).toBeInTheDocument();
 });
+
+test("shows a working indicator with elapsed time and the model-stream tail while working", () => {
+  useSessionStore.getState().applyEvent({ type: "text_delta", text: "Reading the schema for dim_patient" });
+  render(<CentralStatus working />);
+  // elapsed time (e.g. "0s")
+  expect(screen.getByText(/\d+s/)).toBeInTheDocument();
+  // a short tail of the model's current output
+  expect(screen.getByText(/Reading the schema for dim_patient/)).toBeInTheDocument();
+});
+
+test("no working indicator when not working, even with a transcript", () => {
+  useSessionStore.getState().applyEvent({ type: "text_delta", text: "background narration" });
+  render(<CentralStatus working={false} />);
+  expect(screen.queryByText(/\d+s/)).not.toBeInTheDocument();
+});