feat: add Gateway page and Runs page with real-time updates

- Implemented a new Gateway page that provides a uniform ingress for deployed agents.
- Created a Runs page that lists recent pipeline executions with auto-refresh and error handling.
- Added a RunsListener component to handle real-time notifications for new runs via SSE.
- Updated the AppSidebar to include links to the new Gateway and Runs pages.
- Enhanced FlowNode component to improve status display and styling.
- Introduced EmptySection component for consistent empty state presentation.
- Updated API utility to include a new endpoint for the runs stream.

web/app/gateway/page.tsx

@@ -0,0 +1,10 @@
+import { EmptySection } from "@/components/empty-section";
+
+export default function GatewayPage() {
+  return (
+    <EmptySection
+      title="Gateway"
+      description="Public ingress for deployed agents — routes, auth, rate limits, OpenTelemetry export. Replaces the per-runtime gateway (Bedrock AgentCore endpoints, Vertex Reasoning Engine routes, K8s Ingress) with a uniform layer."
+    />
+  );
+}