fix: 调整目录框位置和最大高度以改善布局

Signed-off-by: wangsimiao1 <wangsimiao1@xiaomi.com>

Author: WangSimiao2000

_sass/pages/_post.scss

@@ -261,10 +261,9 @@ header {
 }
 
 #toc-wrapper {
-  top: 0;
-  transition: top 0.2s cubic-bezier(0.22, 1, 0.36, 1);
+  top: 3rem;
   overflow-y: auto;
-  max-height: 100vh;
+  max-height: calc(100vh - 3rem);
   scrollbar-width: none;
   margin-top: 2rem;