feat：重构倒计时相关内容，将倒计时逻辑放在后端进行

src/main/java/com/guo/learningprogresstracker/dto/response/StudySessionResponse.java

@@ -42,4 +42,10 @@ public class StudySessionResponse {
 
     @Schema(description = "学习会话的状态（进行中、暂停、已结束）")
     private String sessionState;
+
+    @Schema(description = "倒计时开始位置（S）")
+    private double pointerPosition;
+
+    @Schema(description = "系统提示")
+    private String systemMessage;
 }