feat: 官网顶部导航栏胶囊高亮效果

- 去掉 border-bottom: 2px solid 蓝色底线
- 改为 background: rgba(255,255,255,0.08) 半透明圆角背景
- 加 border-radius: 8px 胶囊形状
- 平滑 0.2s 过渡

common/common.css

@@ -120,16 +120,19 @@ body {
   display: flex;
   align-items: center;
   height: 56px;
-  padding: 0 20px;
+  padding: 0 16px;
+  margin: 0 2px;
   font-size: 15px;
   cursor: pointer;
-  border-bottom: 2px solid transparent;
-  transition: color 0.2s ease;
+  border-bottom: none;
+  border-radius: 8px;
+  transition: color 0.2s ease, background 0.2s ease;
 }
 
 .nav-menu .nav-item:hover {
   color: #fff;
-  border-bottom: 2px solid rgba(59, 130, 246, 0.8);
+  background: rgba(255, 255, 255, 0.08);
+  border-bottom: none;
 }