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/AppShell.tsx

@@ -176,7 +176,7 @@ export function AppShell() {
           <div className="mx-auto flex max-w-3xl flex-col gap-6">
             {activeSessionId ? (
               <>
-                <CentralStatus />
+                <CentralStatus working={working} />
                 <WidgetHost sessionId={activeSessionId} />
               </>
             ) : (