Fix all frontend ESLint errors

- Add eslint-plugin-react-hooks to package.json and config
- Fix duplicate imports in HostsPage.tsx
- Remove non-null assertion in main.tsx
- Fix != to !== in HostDetailPage.tsx and MaintenanceWindowsPage.tsx
- Fix unused variable in ReportsPage.tsx
- Change console.debug to console.warn in useJobWebSocket.ts

frontend/src/main.tsx

@@ -4,7 +4,9 @@ import { BrowserRouter } from 'react-router-dom'
 import App from './App'
 import './index.css'
 
-ReactDOM.createRoot(document.getElementById('root')!).render(
+const root = document.getElementById('root')
+if (!root) throw new Error('Root element not found')
+ReactDOM.createRoot(root).render(
   <React.StrictMode>
     <BrowserRouter>
       <App />