feat(dashboard): React frontend scaffold with overview page

- Vite + React 18 + TypeScript + Tailwind CSS
- Dashboard overview with health gauge, pillar bars, alerts
- WebSocket hook for real-time updates
- Layout with sidebar navigation and live indicator
- Placeholder pages for Mesh, Environment, Config, Alerts
- Dark theme ops center aesthetic with JetBrains Mono

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

dashboard-frontend/src/main.tsx

@@ -0,0 +1,13 @@
+import React from 'react'
+import ReactDOM from 'react-dom/client'
+import { BrowserRouter } from 'react-router-dom'
+import App from './App'
+import './index.css'
+
+ReactDOM.createRoot(document.getElementById('root')!).render(
+  <React.StrictMode>
+    <BrowserRouter>
+      <App />
+    </BrowserRouter>
+  </React.StrictMode>,
+)