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/hooks/useSSE.ts

@@ -0,0 +1,159 @@
+import { useEffect, useRef, useCallback, useState } from 'react'
+
+/** SSE connection state reported to consumers. */
+export type SSEStatus = 'connecting' | 'connected' | 'reconnecting' | 'error'
+
+/** Typed SSE event received from the backend. */
+export interface SSEMessage {
+  /** event: field from the SSE frame */
+  type: string
+  /** parsed JSON from the data: field */
+  data: unknown
+}
+
+export interface UseSSEOptions {
+  /** Endpoint URL — defaults to /api/events */
+  url?: string
+  /** Called for every SSE message (all event types) */
+  onMessage?: (msg: SSEMessage) => void
+  /** Called when connection opens or reconnects */
+  onOpen?: () => void
+  /** Called on unrecoverable error */
+  onError?: (err: Event) => void
+  /** Base delay in ms before the first reconnect attempt (default 1 000) */
+  reconnectBaseMs?: number
+  /** Maximum reconnect delay in ms (default 30 000) */
+  reconnectMaxMs?: number
+  /** Set false to disable auto-connect (useful in tests) */
+  enabled?: boolean
+}
+
+const SSE_EVENTS = ['agent.status', 'agent.task', 'agent.progress', 'fleet.update', 'connected'] as const
+
+/**
+ * useSSE — mounts a persistent SSE connection to the Control Center backend.
+ *
+ * Handles:
+ *  - Initial connection on mount
+ *  - Exponential back-off reconnection on drop
+ *  - Cleanup on unmount
+ *  - All four event types: agent.status, agent.task, agent.progress, fleet.update
+ */
+export function useSSE({
+  url = '/api/events',
+  onMessage,
+  onOpen,
+  onError,
+  reconnectBaseMs = 1_000,
+  reconnectMaxMs = 30_000,
+  enabled = true,
+}: UseSSEOptions = {}): { status: SSEStatus } {
+  const [status, setStatus] = useState<SSEStatus>('connecting')
+
+  // Stable refs so the effect doesn't need to re-run when callbacks change
+  const onMessageRef = useRef(onMessage)
+  const onOpenRef = useRef(onOpen)
+  const onErrorRef = useRef(onError)
+  onMessageRef.current = onMessage
+  onOpenRef.current = onOpen
+  onErrorRef.current = onError
+
+  const reconnectAttemptRef = useRef(0)
+  const reconnectTimerRef = useRef<ReturnType<typeof setTimeout> | null>(null)
+  const esRef = useRef<EventSource | null>(null)
+  const mountedRef = useRef(true)
+
+  const clearReconnectTimer = useCallback(() => {
+    if (reconnectTimerRef.current !== null) {
+      clearTimeout(reconnectTimerRef.current)
+      reconnectTimerRef.current = null
+    }
+  }, [])
+
+  const connect = useCallback(() => {
+    if (!mountedRef.current || !enabled) return
+
+    // Clean up any existing connection
+    if (esRef.current) {
+      esRef.current.close()
+      esRef.current = null
+    }
+
+    setStatus(reconnectAttemptRef.current === 0 ? 'connecting' : 'reconnecting')
+
+    const es = new EventSource(url)
+    esRef.current = es
+
+    es.onopen = () => {
+      if (!mountedRef.current) return
+      reconnectAttemptRef.current = 0
+      setStatus('connected')
+      onOpenRef.current?.()
+    }
+
+    es.onerror = (evt) => {
+      if (!mountedRef.current) return
+
+      // EventSource auto-retries but we manage our own to get back-off control
+      es.close()
+      esRef.current = null
+
+      onErrorRef.current?.(evt)
+
+      // Exponential back-off: 1s, 2s, 4s … capped at reconnectMaxMs
+      const delay = Math.min(
+        reconnectBaseMs * 2 ** reconnectAttemptRef.current,
+        reconnectMaxMs,
+      )
+      reconnectAttemptRef.current += 1
+      setStatus('reconnecting')
+
+      clearReconnectTimer()
+      reconnectTimerRef.current = setTimeout(() => {
+        if (mountedRef.current) connect()
+      }, delay)
+    }
+
+    // Register listeners for all known event types
+    for (const eventType of SSE_EVENTS) {
+      es.addEventListener(eventType, (evt: MessageEvent) => {
+        if (!mountedRef.current) return
+        let data: unknown = evt.data
+        try {
+          data = JSON.parse(evt.data as string)
+        } catch {
+          // leave as raw string
+        }
+        onMessageRef.current?.({ type: eventType, data })
+      })
+    }
+
+    // Catch-all for unnamed events (type == 'message')
+    es.onmessage = (evt: MessageEvent) => {
+      if (!mountedRef.current) return
+      let data: unknown = evt.data
+      try {
+        data = JSON.parse(evt.data as string)
+      } catch {
+        // leave as raw string
+      }
+      onMessageRef.current?.({ type: 'message', data })
+    }
+  }, [url, enabled, reconnectBaseMs, reconnectMaxMs, clearReconnectTimer])
+
+  useEffect(() => {
+    mountedRef.current = true
+    if (enabled) connect()
+
+    return () => {
+      mountedRef.current = false
+      clearReconnectTimer()
+      if (esRef.current) {
+        esRef.current.close()
+        esRef.current = null
+      }
+    }
+  }, [connect, enabled, clearReconnectTimer])
+
+  return { status }
+}