Add Cortex Portal — web visualization for knowledge graph

Express API server wrapping existing store/graph core with REST endpoints
for nodes, edges, graph, and search. React + Vite portal with React Flow
for interactive graph visualization, Tailwind CSS styling, and full CRUD UI
(sidebar, node panel, add/link modals, search bar, toast notifications).

portal/src/main.tsx

@@ -0,0 +1,17 @@
+import './app.css';
+import { StrictMode } from 'react';
+import { createRoot } from 'react-dom/client';
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
+import App from './App';
+
+const queryClient = new QueryClient({
+  defaultOptions: { queries: { refetchOnWindowFocus: false } },
+});
+
+createRoot(document.getElementById('root')!).render(
+  <StrictMode>
+    <QueryClientProvider client={queryClient}>
+      <App />
+    </QueryClientProvider>
+  </StrictMode>,
+);