feat(会话)：跨页面自动恢复 + 阻止多任务同时进行

后端：
- StudySessionsServiceImpl.getActiveSession(excludeTaskNum):
  查询 ONGOING/PAUSED 状态的活跃会话，可排除指定任务
- Controller 新增 GET /study-sessions/active?excludeTaskNum=

前端：
- router beforeEach 守卫：读取 localStorage 中 activeSession，
  非 start-task 页面时自动重定向恢复
- Study.vue 开始任务按钮：调用 getActiveSession() 检测是否
  有其他任务正在进行，提示用户前往继续
- StartTask.vue loadTaskSession 成功后将活跃会话写入
  localStorage（taskNum/sessionNum/taskName）
- endSession 成功时清除 localStorage 中的 activeSession
- 路由参数 ?resume=true 时页面顶部显示恢复提示已恢复上次

src/api/studySessions.ts

@@ -37,6 +37,13 @@ export const getReportDraft = (sessionNum: string) => {
     return request.get(`/study-sessions/${sessionNum}/report-draft`);
 };
 
+/** 查询当前是否有活跃会话 */
+export const getActiveSession = (excludeTaskNum?: string) => {
+    const params: Record<string, any> = {};
+    if (excludeTaskNum) params.excludeTaskNum = excludeTaskNum;
+    return request.get('/study-sessions/active', params);
+};
+
 /** 分页查询任务的历史残片 */
 export const getTaskFragments = (taskNum: string, page: number, size: number, keyword?: string) => {
     const params: Record<string, any> = { page, size };

src/components/StartTask.vue

@@ -150,6 +150,9 @@ const BREAK_END_KEY = computed(() => `breakEnd_${taskNum}`);
 const isBreak = ref(false);
 const breakAudio = new Audio('/resource/notification.mp3');
 
+// 恢复提示
+const showResumeHint = ref(route.query.resume === "true");
+
 const progress = computed(() => {
   const total = isBreak.value ? 5 * 60 : 25 * 60;
   const remaining = timerMinutes.value * 60 + timerSeconds.value;
@@ -303,6 +306,7 @@ const endTimer = async (content: string) => {
     }
     clear();
     clearBreakState();
+    localStorage.removeItem("activeSession");
     await router.push("/study");
   } catch {
     ElMessage.error("结束会话失败，请重试");
@@ -389,6 +393,13 @@ const loadTaskSession = async () => {
       }
     }
 
+    // 持久化活跃会话（用于跨页面恢复 + 阻止多任务）
+    localStorage.setItem("activeSession", JSON.stringify({
+      taskNum: taskInfo.value.taskNum,
+      sessionNum: taskInfo.value.sessionNum,
+      taskName: taskInfo.value.taskName,
+    }));
+
     if (taskInfo.value.sessionState === "ONGOING") {
       startTimer();
     } else {
@@ -516,6 +527,14 @@ onUnmounted(() => {
 <template>
   <div>
   <section class="start-page">
+    <el-alert
+      v-if="showResumeHint"
+      title="已恢复上次的学习会话，继续学习吧"
+      type="success"
+      show-icon
+      :closable="true"
+      @close="showResumeHint = false"
+    />
     <div class="task-info-bar">
       <el-tag type="success" effect="plain">任务编号：{{ taskInfo.taskNum }}</el-tag>
       <span class="status-text">状态：{{ statusText }}</span>

src/components/Study.vue

@@ -4,6 +4,7 @@ import request from "@/utils/request";
 import router from "@/router";
 import { ElMessage, ElMessageBox } from "element-plus";
 import { getReviewTaskStatsByTask } from "@/api/review";
+import { getActiveSession } from "@/api/studySessions";
 import {
   getPriorityWeights,
   getTaskApplications,
@@ -174,12 +175,30 @@ watch(selectedTaskId, (newId) => {
   }
 });
 
-const startTask = () => {
+const startTask = async () => {
   if (!selectedTask.value) {
     ElMessage.warning("请先选择一个任务");
     return;
   }
-  router.push(`/start-task/${encodeURIComponent(selectedTask.value.taskNum)}`);
+  const taskNum = selectedTask.value.taskNum;
+  try {
+    const res = await getActiveSession(taskNum);
+    if (res?.code === 200 && res.data) {
+      // 有其他任务的活跃会话
+      const active = res.data;
+      ElMessageBox.confirm(
+        `你还有一个未完成的学习任务「${active.taskName}」（${active.sessionNum}），是否前往继续？`,
+        "有其他任务正在进行",
+        { confirmButtonText: "前往继续", cancelButtonText: "取消", type: "warning" }
+      ).then(() => {
+        router.push(`/start-task/${encodeURIComponent(active.taskNum)}`);
+      }).catch(() => {});
+      return;
+    }
+  } catch {
+    // 查询失败不阻塞，直接跳转
+  }
+  router.push(`/start-task/${encodeURIComponent(taskNum)}`);
 };
 
 const addTask = () => {

src/router/index.ts

@@ -89,6 +89,18 @@ router.beforeEach((to) => {
   if (!isLoggedIn) {
     return "/login";
   }
+  // 如果有活跃会话且未处于 start-task 页面，自动恢复
+  if (!to.path.startsWith("/start-task")) {
+    const stored = localStorage.getItem("activeSession");
+    if (stored) {
+      try {
+        const { taskNum } = JSON.parse(stored);
+        if (taskNum) {
+          return `/start-task/${encodeURIComponent(taskNum)}?resume=true`;
+        }
+      } catch {}
+    }
+  }
   return true;
 });