CUB-125: implement real-time SSE/WebSocket in React frontend

- Add useSSE hook with exponential back-off reconnect (1s → 30s)
- Add useRealtimeSync hook: maps SSE events to React Query invalidation
  (agent.status → agents; agent.task/agent.progress → tasks+agents; fleet.update → all)
- Add SSEContext/SSEProvider so connection status is available app-wide
- Mount SSEProvider in main.tsx inside QueryClientProvider (no polling)
- Show live/connecting/reconnecting/disconnected badge in sidebar + mobile header
- Update SettingsPage: replace polling interval UI with SSE status panel
- Disable React Query polling (staleTime 60s); all updates pushed via SSE

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/main.tsx

@@ -4,13 +4,16 @@ import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
 import { BrowserRouter } from 'react-router-dom'
 import ErrorBoundary from './components/ErrorBoundary'
 import { ThemeProvider } from './hooks/useTheme'
+import { SSEProvider } from './contexts/SSEContext'
 import './index.css'
 import App from './App'
 
 const queryClient = new QueryClient({
   defaultOptions: {
     queries: {
-      staleTime: 30_000,
+      // No polling — real-time updates come through SSE.
+      // staleTime is kept high; data is pushed, not pulled.
+      staleTime: 60_000,
       refetchOnWindowFocus: false,
       retry: 1,
     },
@@ -22,9 +25,13 @@ createRoot(document.getElementById('root')!).render(
     <ErrorBoundary>
       <ThemeProvider>
         <QueryClientProvider client={queryClient}>
-          <BrowserRouter>
-            <App />
-          </BrowserRouter>
+          {/* SSEProvider must live inside QueryClientProvider so it can call
+              useQueryClient() to invalidate caches on incoming events. */}
+          <SSEProvider>
+            <BrowserRouter>
+              <App />
+            </BrowserRouter>
+          </SSEProvider>
         </QueryClientProvider>
       </ThemeProvider>
     </ErrorBoundary>