CUB-121: build React pages with real API integration

- HubPage: agent summary stats, cards, status badges, progress bars, refresh
- LogsPage: activity feed from tasks, status filter, loading skeleton
- ProjectsPage: project cards with status badges and agent counts
- SessionsPage: responsive table/card view with model/token info
- SettingsPage: dark mode toggle, gateway URL, refresh interval persist
- ThemeProvider with dark/light mode via CSS custom properties
- useLocalStorage hook for settings persistence
- Loading/error/empty states across all pages
- npm run build passes cleanly

frontend/src/main.tsx

@@ -3,6 +3,7 @@ import { createRoot } from 'react-dom/client'
 import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
 import { BrowserRouter } from 'react-router-dom'
 import ErrorBoundary from './components/ErrorBoundary'
+import { ThemeProvider } from './hooks/useTheme'
 import './index.css'
 import App from './App'
 
@@ -19,11 +20,13 @@ const queryClient = new QueryClient({
 createRoot(document.getElementById('root')!).render(
   <StrictMode>
     <ErrorBoundary>
-      <QueryClientProvider client={queryClient}>
-        <BrowserRouter>
-          <App />
-        </BrowserRouter>
-      </QueryClientProvider>
+      <ThemeProvider>
+        <QueryClientProvider client={queryClient}>
+          <BrowserRouter>
+            <App />
+          </BrowserRouter>
+        </QueryClientProvider>
+      </ThemeProvider>
     </ErrorBoundary>
   </StrictMode>,
 )