refactor(study): 移除调试日志并优化视频时间获取逻辑

在ExamTest组件中移除已注释的调试日志
在courseDetail组件中增加视频元素存在性检查，避免潜在错误

src/components/study/ExamTest.vue

@@ -590,7 +590,7 @@ export default {
   },
 
   mounted() {
-    console.log(this.loginInfo);
+    // console.log(this.loginInfo);
 
     this.start();
   },

src/components/study/courseDetail.vue

@@ -357,8 +357,12 @@ export default {
       // 定时器
       function timer() {
         // console.log("被调用了");
+        const video = document.getElementById("video");
+        let curTime;
+        if(video) {
+          curTime = video.currentTime;
+        }
 
-        let curTime = document.getElementById("video").currentTime;
         let apartTime = curTime - _this.current_sign;
         if (apartTime > 2 && _this.section[_this.choose_section_index].is_end != 1) {
           document.getElementById("video").currentTime = _this.current_sign;