feat(notifications): poll real PerformanceAlert rows from sidecar

usePerformanceAlerts now fetches /api/supervisor/performance-alerts
every 60s instead of computing client-side. Dismiss + dismiss-all hit
the sidecar so state survives reload. Toast fires when new alerts arrive.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/layout/notification-bell.tsx

@@ -91,7 +91,7 @@ export const NotificationBell = () => {
                                         <p className="text-sm font-medium text-primary">{alert.agent}</p>
                                         <p className="text-xs text-tertiary">{alert.type}</p>
                                     </div>
-                                    <Badge size="sm" color={alert.severity} type="pill-color">{alert.value}</Badge>
+                                    <Badge size="sm" color={alert.severity === 'error' ? 'error' : alert.severity === 'warning' ? 'warning' : 'gray'} type="pill-color">{alert.value}</Badge>
                                     <button
                                         onClick={() => dismiss(alert.id)}
                                         className="text-fg-quaternary hover:text-fg-secondary shrink-0 transition duration-100 ease-linear"