feat(会话)：新增活跃会话查询接口

StudySessionsServiceImpl.getActiveSession(excludeTaskNum)：
- 查询所有 ONGOING/PAUSED 状态的 session
- 如果活跃会话属于 excludeTaskNum 则返回 null（同一任务继续）
- 否则返回活跃会话的 StudySessionResponse

Controller 新增 GET /study-sessions/active?excludeTaskNum=
用于前端页面恢复 + 阻止多任务同时开始

src/main/java/com/guo/learningprogresstracker/controller/StudySessionController.java

@@ -130,4 +130,11 @@ public class StudySessionController {
             @RequestParam(required = false) String keyword) {
         return CommonResult.success(studySessionsServiceImpl.getTaskReports(taskNum, page, size, keyword));
     }
+
+    /** 查询当前是否有活跃会话（用于跨页面恢复 + 阻止多任务） */
+    @GetMapping("/active")
+    public CommonResult<StudySessionResponse> getActiveSession(
+            @RequestParam(required = false) String excludeTaskNum) {
+        return CommonResult.success(studySessionsServiceImpl.getActiveSession(excludeTaskNum));
+    }
 }

src/main/java/com/guo/learningprogresstracker/service/StudySessionsService.java

@@ -33,4 +33,6 @@ public interface StudySessionsService extends IService<StudySessionsEntity> {
     Page<StudyReportFragmentsEntity> getTaskFragments(String taskNum, int page, int size, String keyword);
 
     Page<StudyReportsEntity> getTaskReports(String taskNum, int page, int size, String keyword);
+
+    StudySessionResponse getActiveSession(String excludeTaskNum);
 }

src/main/java/com/guo/learningprogresstracker/service/impl/StudySessionsServiceImpl.java

@@ -217,4 +217,39 @@ public class StudySessionsServiceImpl extends ServiceImpl<StudySessionsMapper, S
         return studyReportsMapper.selectPage(pg, wrapper);
     }
 
+    // ============ 活跃会话查询 ============
+
+    /**
+     * 查询当前用户是否有活跃会话（ONGOING 或 PAUSED）。
+     *
+     * @param excludeTaskNum 可选，排除指定任务（同一任务继续学习时不会视为冲突）
+     * @return 活跃会话响应；无活跃会话时返回 null
+     */
+    @Override
+    public StudySessionResponse getActiveSession(String excludeTaskNum) {
+        var wrapper = Wrappers.lambdaQuery(StudySessionsEntity.class)
+                .in(StudySessionsEntity::getSessionState,
+                        StudySessionStateEnum.ONGOING.name(),
+                        StudySessionStateEnum.PAUSED.name())
+                .orderByDesc(StudySessionsEntity::getCreatedTime)
+                .last("LIMIT 1");
+
+        StudySessionsEntity session = this.getOne(wrapper);
+        if (session == null) return null;
+
+        // 如果活跃会话属于被排除的任务，视为无冲突
+        if (excludeTaskNum != null && excludeTaskNum.equals(session.getTaskNum())) {
+            return null;
+        }
+
+        String taskName = tasksServiceImpl.getOneOpt(
+                        Wrappers.lambdaQuery(TaskEntity.class).eq(TaskEntity::getTaskNum, session.getTaskNum()))
+                .map(TaskEntity::getTaskName)
+                .orElse(null);
+
+        StudySessionResponse response = StudySessionConvert.MAPPER.toStudySessionResponse(session);
+        response.setTaskName(taskName);
+        return response;
+    }
+
 }