task-86: 将隐藏页面轮询间隔、前台恢复和退避策略统一配置化

task-progress-config 新增统一配置 API：getTaskPollBackoffMs(attempt)
指数退避（base×2^attempt 封顶 max）、前台恢复开关与延迟
（getTaskForegroundRefreshEnabled/DelayMs）、configureTaskPolling/
resetTaskPollingConfig 运行时配置。校验失败抛错且零状态变更，
document 缺失时按可见间隔降级。useTaskProgressLoop 改用退避配置
替代硬编码 500ms 重试，前台恢复走可配置开关与延迟（含 in-flight 防
并发）。8 个测试覆盖默认、多配置、幂等、空、单字段、封顶、非法输入
与故障恢复。

frontend-vue/src/shared/composables/useTaskProgressLoop.ts

@@ -1,5 +1,10 @@
 import { onBeforeUnmount, ref, watch, type Ref } from 'vue'
-import { getTaskPollIntervalMs } from '@/shared/task-progress-config'
+import {
+  getTaskPollIntervalMs,
+  getTaskPollBackoffMs,
+  getTaskForegroundRefreshEnabled,
+  getTaskForegroundRefreshDelayMs,
+} from '@/shared/task-progress-config'
 import { createCategorizedTimers } from '@/shared/utils/categorized-timers'
 import {
   createTaskPollingBaseline,
@@ -233,8 +238,8 @@ export function useTaskProgressLoop<TDetail>(
       if (disposed) return
       if (!taskIds.value.length) return
       if (inFlight.value) {
-        // 上一次还没回，500ms 后再试
-        pollTimer = timers.setTimeout('task-poll', run, 500)
+        // 上一次还没回，按退避策略延迟重试（默认 500ms，可配置）
+        pollTimer = timers.setTimeout('task-poll', run, getTaskPollBackoffMs(0))
         return
       }
       await refreshOnce()
@@ -277,17 +282,44 @@ export function useTaskProgressLoop<TDetail>(
     { flush: 'post' },
   )
 
-  // 切到前台后立刻拉一次，让用户回到页面看到的是最新状态
+  // 切到前台后立刻拉一次（可配置开关与延迟），让用户回到页面看到的是最新状态
   let visibilityHandler: (() => void) | null = null
   if (typeof document !== 'undefined') {
     visibilityHandler = () => {
-      if (document.visibilityState === 'visible' && taskIds.value.length > 0) {
-        scheduleNext(true)
+      if (
+        getTaskForegroundRefreshEnabled() &&
+        document.visibilityState === 'visible' &&
+        taskIds.value.length > 0
+      ) {
+        const delay = getTaskForegroundRefreshDelayMs()
+        if (delay > 0) {
+          scheduleNextDelayed(delay)
+        } else {
+          scheduleNext(true)
+        }
       }
     }
     document.addEventListener('visibilitychange', visibilityHandler)
   }
 
+  function scheduleNextDelayed(delayMs: number) {
+    if (disposed || pollTimer != null) return
+    clearPollTimer()
+    const run = () => {
+      pollTimer = null
+      if (disposed || !taskIds.value.length) return
+      if (inFlight.value) {
+        pollTimer = timers.setTimeout('task-poll', run, getTaskPollBackoffMs(0))
+        return
+      }
+      void refreshOnce()
+      if (!disposed && taskIds.value.length > 0) {
+        pollTimer = timers.setTimeout('task-poll', run, intervalMs())
+      }
+    }
+    pollTimer = timers.setTimeout('task-poll', run, delayMs)
+  }
+
   function dispose() {
     if (disposed) return
     disposed = true