refactor(frontend): unify polling via usePolling + visibility pause (S6)

- usePolling gains a pauseWhenHidden option (default true): the timer
  pauses while the document is hidden (background tab / minimised) and
  resumes with an immediate reload on returning to visible. Callers keep
  owning their initial fetch, so no duplicate first request.
- CronPoolMonitor now delegates its 5s poll to the shared usePolling hook
  (initial load stays in its own useEffect); removes the bespoke
  setInterval + manual visibility gap.
- CronPoolMonitor.test: visibility guard now passes (3/3), removing the
  previous it.fails. Frontend suite: 18/18; tsc clean; backend 878.

Resolves scan §6.3.

.scratch/v111-architecture-scan.md

@@ -113,7 +113,7 @@ except Exception as e:
 
 **当前状态**：两个 xfail 守卫在 `test_openclaw_client_and_webhook.py`，CI 不阻塞；修复后移除 xfail 即转绿。
 
-### 6.3 前端 CronPoolMonitor 不响应 visibilitychange（S6 缺口，T2 暴露）
+### 6.3 前端 CronPoolMonitor 不响应 visibilitychange（S6 缺口，T2 暴露）— ✅ RESOLVED (S6 重构)
 
 **症状**：`frontend/web/src/pages/CronPoolMonitor.test.tsx` 的 `it.fails('listens to visibilitychange ...')`。
 CronPoolMonitor 当前的 `useEffect` 只做了 `setInterval(loadData, 5000)` + `clearInterval` 清理，不监听 `document.visibilitychange`。

frontend/web/src/hooks/usePolling.ts

@@ -5,14 +5,70 @@ import { useEffect, useRef } from 'react'
  * when it flips false or the component unmounts. The latest `fn` is always
  * used without restarting the interval, so callers can pass a fresh closure
  * each render (e.g. `() => reload(true)`) without churning the timer.
+ *
+ * S6: when ``pauseWhenHidden`` is true (default) the timer pauses while
+ * the document is hidden (background tab, minimised) and resumes
+ * immediately — calling ``fn`` once on the way back to ``visible`` so
+ * the displayed data is fresh. The ``visibilitychange`` listener is added
+ * on mount and removed on unmount.
  */
-export function usePolling(fn: () => void, ms: number, enabled: boolean): void {
+export function usePolling(
+  fn: () => void,
+  ms: number,
+  enabled: boolean,
+  options: { pauseWhenHidden?: boolean } = {},
+): void {
+  const pauseWhenHidden = options.pauseWhenHidden ?? true
   const fnRef = useRef(fn)
   fnRef.current = fn
 
   useEffect(() => {
     if (!enabled) return
-    const id = window.setInterval(() => fnRef.current(), ms)
-    return () => window.clearInterval(id)
-  }, [enabled, ms])
+
+    let intervalId: ReturnType<typeof setInterval> | null = null
+    const start = () => {
+      if (intervalId !== null) return
+      intervalId = setInterval(() => fnRef.current(), ms)
+    }
+    const stop = () => {
+      if (intervalId !== null) {
+        clearInterval(intervalId)
+        intervalId = null
+      }
+    }
+
+    const startWithFreshCall = () => {
+      fnRef.current()
+      start()
+    }
+
+    const onVisibilityChange = () => {
+      if (document.visibilityState === 'visible') {
+        // Returning to the visible tab: refresh immediately and resume the
+        // timer so the user sees up-to-date data right away.
+        startWithFreshCall()
+      } else {
+        stop()
+      }
+    }
+
+    // Start the timer when enabled. Callers own the initial fetch (most
+    // hooks already issue one in their own useEffect), so we do NOT call fn
+    // here — that avoids a duplicate initial request. The hook is only
+    // responsible for the interval and (optionally) the visibility pause.
+    if (!pauseWhenHidden || document.visibilityState === 'visible') {
+      start()
+    }
+
+    if (pauseWhenHidden) {
+      document.addEventListener('visibilitychange', onVisibilityChange)
+    }
+
+    return () => {
+      stop()
+      if (pauseWhenHidden) {
+        document.removeEventListener('visibilitychange', onVisibilityChange)
+      }
+    }
+  }, [enabled, ms, pauseWhenHidden])
 }

frontend/web/src/pages/CronPoolMonitor.test.tsx

@@ -97,14 +97,30 @@ describe('CronPoolMonitor polling lifecycle', () => {
     const after = vi.mocked(openclawCronPoolApi.getStatus).mock.calls.length
     expect(after).toBe(before)
   })
-  it.fails('listens to visibilitychange to pause polling when tab is hidden', async () => {
-    const addSpy = vi.spyOn(document, 'addEventListener')
+  it('pauses polling while the document is hidden and resumes on visible', async () => {
+    // Default starting state is visible.
+    vi.spyOn(document, 'visibilityState', 'get').mockReturnValue('visible')
+
     render(<CronPoolMonitor />)
     await settle()
-    const hasVisibility = addSpy.mock.calls.some(
-      (call: unknown[]) => call[0] === 'visibilitychange',
-    )
-    expect(hasVisibility).toBe(true)
-    addSpy.mockRestore()
+    const initial = vi.mocked(openclawCronPoolApi.getStatus).mock.calls.length
+    expect(initial).toBeGreaterThanOrEqual(1)
+
+    // While hidden, advancing the clock must NOT trigger another poll.
+    vi.spyOn(document, 'visibilityState', 'get').mockReturnValue('hidden')
+    document.dispatchEvent(new Event('visibilitychange'))
+    await act(async () => {
+      await vi.advanceTimersByTimeAsync(15000)
+    })
+    await settle()
+    const afterHidden = vi.mocked(openclawCronPoolApi.getStatus).mock.calls.length
+    expect(afterHidden).toBe(initial)
+
+    // Returning to visible resumes polling and triggers an immediate reload.
+    vi.spyOn(document, 'visibilityState', 'get').mockReturnValue('visible')
+    document.dispatchEvent(new Event('visibilitychange'))
+    await settle()
+    const afterVisible = vi.mocked(openclawCronPoolApi.getStatus).mock.calls.length
+    expect(afterVisible).toBeGreaterThan(initial)
   })
 })

frontend/web/src/pages/CronPoolMonitor.tsx

@@ -5,6 +5,7 @@ import {
 import type { ColumnsType } from 'antd/es/table'
 import { ReloadOutlined, WarningOutlined } from '@ant-design/icons'
 import { openclawCronPoolApi, type CronPoolAlert, type CronPoolMetrics, type CronPoolStatus } from '../api'
+import { usePolling } from '../hooks/usePolling'
 import { colors } from '../tokens'
 import { formatDateTime } from '../utils/date'
 
@@ -34,12 +35,17 @@ export default function CronPoolMonitor() {
     }
   }
 
+  // Initial fetch (usePolling owns the 5s interval + visibility pause).
   useEffect(() => {
     void loadData()
-    const interval = setInterval(() => void loadData(), 5000)
-    return () => clearInterval(interval)
+    // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [])
 
+  // S6: defer the recurring 5s poll to the shared `usePolling` hook. The
+  // default pauses the timer while the document is hidden (background tab /
+  // minimised window) and re-fetches once on the way back to `visible`.
+  usePolling(() => { void loadData() }, 5000, true)
+
   const handleScale = async () => {
     setScaleBusy(true)
     try {