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/pages/SettingsPage.tsx

@@ -1,18 +1,36 @@
 import { useTheme } from '../hooks/useTheme'
 import { useLocalStorage } from '../hooks/useLocalStorage'
-import { Sun, Moon, Monitor, Zap, Clock } from 'lucide-react'
+import { useSSEContext } from '../contexts/SSEContext'
+import { Sun, Moon, Monitor, Zap, Radio } from 'lucide-react'
 
-const REFRESH_PRESETS = [
-  { label: '5s', value: 5_000 },
-  { label: '10s', value: 10_000 },
-  { label: '30s', value: 30_000 },
-  { label: '60s', value: 60_000 },
-]
+const SSE_STATUS_COPY: Record<string, { label: string; description: string; color: string }> = {
+  connected: {
+    label: 'Connected',
+    description: 'Real-time updates are active. Agent status, tasks, and progress stream live.',
+    color: 'text-green-500',
+  },
+  connecting: {
+    label: 'Connecting…',
+    description: 'Establishing SSE connection to the backend.',
+    color: 'text-yellow-500',
+  },
+  reconnecting: {
+    label: 'Reconnecting…',
+    description: 'Connection lost. Retrying with exponential back-off.',
+    color: 'text-yellow-500',
+  },
+  error: {
+    label: 'Disconnected',
+    description: 'Could not connect to the SSE endpoint. Check that the backend is reachable.',
+    color: 'text-red-500',
+  },
+}
 
 export default function SettingsPage() {
   const { isDark, toggleTheme } = useTheme()
   const [gatewayUrl, setGatewayUrl] = useLocalStorage('cc-gateway-url', '')
-  const [refreshInterval, setRefreshInterval] = useLocalStorage('cc-refresh-interval', 30_000)
+  const { sseStatus } = useSSEContext()
+  const sseInfo = SSE_STATUS_COPY[sseStatus] ?? SSE_STATUS_COPY.error
 
   return (
     <div className="space-y-8 max-w-2xl">
@@ -80,45 +98,31 @@ export default function SettingsPage() {
         </div>
       </section>
 
-      {/* Refresh */}
+      {/* Real-time connection status */}
       <section className="space-y-4">
         <h2 className="text-lg font-semibold flex items-center gap-2">
-          <Clock size={20} className="text-primary" />
-          Auto Refresh
+          <Radio size={20} className="text-primary" />
+          Real-time Updates
         </h2>
 
         <div className="p-5 rounded-xl border border-surface-light bg-surface-dark space-y-3">
-          <p className="text-sm text-on-surface-variant">Data refresh interval for agent status and logs</p>
-
-          <div className="flex flex-col gap-2">
-            <input
-              type="range"
-              min="0"
-              max="3"
-              step="1"
-              value={REFRESH_PRESETS.findIndex((p) => p.value === refreshInterval)}
-              onChange={(e) => {
-                const idx = parseInt(e.target.value)
-                setRefreshInterval(REFRESH_PRESETS[idx].value)
-              }}
-              className="w-full accent-primary"
-            />
-            <div className="flex justify-between text-xs text-on-surface-muted">
-              {REFRESH_PRESETS.map((p) => (
-                <button
-                  key={p.label}
-                  onClick={() => setRefreshInterval(p.value)}
-                  className={`px-3 py-1 rounded-lg transition-colors ${
-                    refreshInterval === p.value
-                      ? 'bg-primary/10 text-primary'
-                      : 'hover:bg-surface-light'
-                  }`}
-                >
-                  {p.label}
-                </button>
-              ))}
+          <div className="flex items-center justify-between">
+            <div>
+              <p className="font-medium">SSE Connection</p>
+              <p className="text-sm text-on-surface-variant mt-0.5">{sseInfo.description}</p>
             </div>
+            <span className={`text-sm font-semibold whitespace-nowrap ${sseInfo.color}`}>
+              {sseInfo.label}
+            </span>
           </div>
+          <p className="text-xs text-on-surface-muted">
+            Endpoint: <code className="bg-surface-light px-1.5 py-0.5 rounded text-on-surface-variant">/api/events</code>
+            &nbsp;·&nbsp;Events: agent.status, agent.task, agent.progress, fleet.update
+          </p>
+          <p className="text-xs text-on-surface-muted">
+            Polling is disabled. All status updates are pushed from the server over a persistent SSE connection.
+            The client reconnects automatically with exponential back-off on drop.
+          </p>
         </div>
       </section>
     </div>