feat(style): 超级菜单下拉展示动画 + 切换菜单渐隐渐显（v-show 过渡）

src/layouts/SiteLayout.vue

@@ -162,7 +162,7 @@
         :key="mm.key"
         :id="'mega-menu-' + mm.key"
         class="mega-menu fixed top-16 left-0 right-0 bg-white menu-shadow z-50 overflow-y-auto md:max-h-[75vh]"
-        :class="activeMega === mm.key ? 'mega-menu-visible' : 'hidden'"
+        v-show="activeMega === mm.key"
       >
         <div class="w-full flex flex-col md:flex-row">
           <aside class="w-full md:w-80 bg-sidebar p-8">
@@ -465,17 +465,16 @@ watch(
   transition: all 0.3s;
 }
 
-/* Mega 菜单渐显动画 */
-.mega-menu {
+/* Mega 菜单：hover nav 后从 nav 后方下拉展示；切换菜单渐隐渐显（v-show 过渡） */:global(.v-enter-active),:global(.v-leave-active) {
+  transition: opacity 0.25s ease, transform 0.3s ease;
+}
+:global(.v-enter-from) {
+  opacity: 0;
+  transform: translateY(-16px);
+}
+:global(.v-leave-to) {
   opacity: 0;
   transform: translateY(-8px);
-  pointer-events: none;
-  transition: opacity 0.25s ease, transform 0.25s ease;
-}
-.mega-menu-visible {
-  opacity: 1;
-  transform: translateY(0);
-  pointer-events: auto;
 }
 
 /* 搜索框 hover/focus 平滑变宽：右侧固定，左侧向左延申至近似居中，贝塞尔曲线过渡 */