feat(toc): add sticky positioning and entry animation for improved UX

src/styles/header-dividing.scss

@@ -10,6 +10,10 @@
   --dash-spacing: 10px;
   --dash-width: 2px;
 
+  // 进入动画初始状态（仅 sticky 激活时播放）
+  opacity: 1;
+  transition: opacity 0.3s ease-in-out;
+
   // 虚线背景层 - 带遮罩
   &::before {
     content: '';
@@ -81,6 +85,10 @@
     opacity: 0.8;
     box-shadow: 0 2px 8px rgba(0, 0, 0, 0.1);
 
+    /* 进入动画：透明度由0到1，宽度由中间到100% */
+    animation: sticky-in 0.5s ease-in-out forwards;
+    opacity: 1;
+
     &::before {
       animation: line-flow 1.5s linear infinite;
       mask-image: none;
@@ -126,4 +134,13 @@
     left: 80%;
     opacity: 0;
   }
-}
+}
+
+@keyframes sticky-in {
+  0% {
+    opacity: 0;
+  }
+  100% {
+    opacity: 1;
+  }
+}