refactor(统计面板): 抽出 useHiddenStudents，两份逐字相同的「请假隐藏」合成一份

StatisticsPanel.vue 和 FlowchartStatisticsPanel.vue 各写了一遍隐藏学生的逻辑
（loadHidden / saveHidden / hideStudent / showAll / onMounted 里的过期清理），除了
存储键和一个参数名逐字相同；「这个人有没有被隐藏」的判断两边还各自内联了三处。

现在是 shared/composables/hiddenStudents.ts，存储键作参数传进去 —— 两个面板仍然各用
各的键，提交统计里隐掉的人不该连带在流程图统计里也消失，那是两件事。

composable 不导出 hiddenStudents 那张表本身，只给 isHidden / notHidden：两个面板要的
都是「这个人该不该显示」，把表递出去只会让判断逻辑又散回组件里。

## 验证

起全栈在浏览器里真点了一遍（提交列表 → 数据统计 → 未完成 tab）：

- 打开「请假隐藏」开关后学生标签变成可关闭，关掉「单田芳」，未完成从 5 变 4、
  「还没交」也同步从 5 变 4；
- localStorage 落的是 {"ks251单田芳": <时间戳>}，刷新页面后仍在，到期时间 120 分钟；
- 流程图那把键 oj_hidden_students_flowchart 全程为 null，没被串到。

vue-tsc、vite build 通过。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012j1vgeDqay8wKCh8dPgPcH

apps/web/src/shared/components/FlowchartStatisticsPanel.vue

@@ -159,6 +159,7 @@ import { formatISO, sub, type Duration } from "date-fns"
 import type { FlowchartStatistics } from "@oj2/contract"
 import { getFlowchartStatistics } from "oj/api"
 import { DURATION_OPTIONS, FLOWCHART_CRITERIA_ORDER } from "utils/constants"
+import { useHiddenStudents } from "../composables/hiddenStudents"
 import { Doughnut, Radar, Bar } from "vue-chartjs"
 import {
   Chart as ChartJS,
@@ -237,66 +238,19 @@ const hasResult = computed(
 const wordcloudCanvas = useTemplateRef<HTMLCanvasElement>("wordcloudCanvas")
 let wordcloudChart: ChartJS | null = null
 
-const HIDE_DURATION = 2 * 60 * 60 * 1000
-const STORAGE_KEY = "oj_hidden_students_flowchart"
+const { hideMode, hideStudent, showAll, isHidden, notHidden } =
+  useHiddenStudents("oj_hidden_students_flowchart")
 
-function loadHidden(): Record<string, number> {
-  try {
-    return JSON.parse(localStorage.getItem(STORAGE_KEY) ?? "{}")
-  } catch {
-    return {}
-  }
-}
+const visibleUnaccepted = computed(() => data.dataUnaccepted.filter(notHidden))
 
-const hiddenStudents = ref<Record<string, number>>(loadHidden())
-const hideMode = ref(false)
-
-function saveHidden(d: Record<string, number>) {
-  localStorage.setItem(STORAGE_KEY, JSON.stringify(d))
-}
-
-function hideStudent(username: string) {
-  hiddenStudents.value = {
-    ...hiddenStudents.value,
-    [username]: Date.now() + HIDE_DURATION,
-  }
-  saveHidden(hiddenStudents.value)
-}
-
-function showAll() {
-  hiddenStudents.value = {}
-  saveHidden({})
-}
-
-const visibleUnaccepted = computed(() => {
-  const now = Date.now()
-  return data.dataUnaccepted.filter((item) => {
-    const exp = hiddenStudents.value[item.username]
-    return !exp || exp <= now
-  })
-})
-
-const hiddenCount = computed(() => {
-  const now = Date.now()
-  return data.dataUnaccepted.filter((item) => {
-    const exp = hiddenStudents.value[item.username]
-    return !!exp && exp > now
-  }).length
-})
+const hiddenCount = computed(
+  () => data.dataUnaccepted.filter((item) => isHidden(item.username)).length,
+)
 
 const adjustedPersonCount = computed(() =>
   Math.max(0, data.personCount - hiddenCount.value),
 )
 
-onMounted(() => {
-  const now = Date.now()
-  const cleaned = Object.fromEntries(
-    Object.entries(hiddenStudents.value).filter(([, exp]) => exp > now),
-  )
-  hiddenStudents.value = cleaned
-  saveHidden(cleaned)
-})
-
 const completionRate = computed(() => {
   if (adjustedPersonCount.value <= 0) return "0%"
   const rate = Math.min(