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/components/Layout.tsx

@@ -1,6 +1,8 @@
 import { useState } from 'react'
 import { NavLink } from 'react-router-dom'
-import { Command, Activity, FolderKanban, Monitor, Settings, Menu, X } from 'lucide-react'
+import { Command, Activity, FolderKanban, Monitor, Settings, Menu, X, Wifi, WifiOff, Loader } from 'lucide-react'
+import { useSSEContext } from '../contexts/SSEContext'
+import type { SSEStatus } from '../hooks/useSSE'
 
 const navItems = [
   { to: '/', icon: Command, label: 'Hub' },
@@ -10,9 +12,29 @@ const navItems = [
   { to: '/settings', icon: Settings, label: 'Settings' },
 ]
 
+/** Small status pill shown in the sidebar footer and mobile header. */
+function SSEStatusBadge({ status, showLabel = false }: { status: SSEStatus; showLabel?: boolean }) {
+  const cfg = {
+    connected: { icon: Wifi, color: 'text-green-500', label: 'Live' },
+    connecting: { icon: Loader, color: 'text-yellow-500 animate-spin', label: 'Connecting' },
+    reconnecting: { icon: Loader, color: 'text-yellow-500 animate-spin', label: 'Reconnecting' },
+    error: { icon: WifiOff, color: 'text-red-500', label: 'Disconnected' },
+  }[status]
+
+  const Icon = cfg.icon
+
+  return (
+    <div className="flex items-center gap-1.5" title={cfg.label}>
+      <Icon size={14} className={cfg.color} />
+      {showLabel && <span className={`text-xs ${cfg.color}`}>{cfg.label}</span>}
+    </div>
+  )
+}
+
 export default function Layout({ children }: { children: React.ReactNode }) {
   const [expanded, setExpanded] = useState(false)
   const [mobileOpen, setMobileOpen] = useState(false)
+  const { sseStatus } = useSSEContext()
 
   return (
     <div className="flex min-h-screen bg-surface-darkest text-on-surface">
@@ -46,6 +68,15 @@ export default function Layout({ children }: { children: React.ReactNode }) {
             </NavLink>
           ))}
         </nav>
+        {/* SSE connection status — footer of sidebar */}
+        <div className="px-4 py-3 border-t border-surface-light flex items-center gap-2">
+          <SSEStatusBadge status={sseStatus} />
+          {expanded && (
+            <span className="text-xs text-on-surface-muted whitespace-nowrap">
+              {sseStatus === 'connected' ? 'Live updates on' : sseStatus}
+            </span>
+          )}
+        </div>
       </aside>
 
       {/* Mobile Header + Bottom Nav */}
@@ -54,6 +85,7 @@ export default function Layout({ children }: { children: React.ReactNode }) {
           <div className="flex items-center gap-2">
             <Command size={22} className="text-primary" />
             <span className="font-bold">Control Center</span>
+            <SSEStatusBadge status={sseStatus} />
           </div>
           <button onClick={() => setMobileOpen(!mobileOpen)} className="p-2">
             {mobileOpen ? <X size={22} /> : <Menu size={22} />}