fix(study): 优化视频定时器处理并添加路由离开清理

确保在组件销毁和路由离开时都清除视频定时器，避免内存泄漏。简化视频当前时间获取逻辑，移除不必要的条件判断。

src/components/study/courseDetail.vue

@@ -357,11 +357,7 @@ export default {
       // 定时器
       function timer() {
         // console.log("被调用了");
-        const video = document.getElementById("video");
+        let curTime = document.getElementById("video").currentTime;
-        let curTime;
-        if(video) {
-          curTime = video.currentTime;
-        }
 
         let apartTime = curTime - _this.current_sign;
         if (apartTime > 2 && _this.section[_this.choose_section_index].is_end != 1) {
@@ -872,6 +868,14 @@ export default {
       }
     },
   },
+  beforeRouteLeave(to, from, next) {
+    // 确保离开路由时清除定时器
+    if (this.video_timer) {
+      clearInterval(this.video_timer);
+      this.video_timer = null; // 重置定时器变量
+    }
+    next();
+  },
   beforeDestroy() {
     if (this.video_timer) {
       clearInterval(this.video_timer);