style(header): 移除顶栏底边线，只保留阴影

去掉 border-b（含 hover 与 menu-open 两处 border 覆盖），保留 shadow-sm 作为层次分隔。

src/layouts/SiteLayout.vue

@@ -5,10 +5,10 @@
     <!-- ===== 顶部导航（对齐原站 _base） ===== -->
     <header
       id="header"
-      class="fixed top-0 left-0 right-0 z-50 transition-all duration-300 ease-in-out bg-white shadow-sm border-b border-gray-100 dark:bg-gray-900 dark:border-gray-800"
+      class="fixed top-0 left-0 right-0 z-50 transition-all duration-300 ease-in-out bg-white shadow-sm dark:bg-gray-900"
       :class="[
         navTheme === 'light'
-          ? '!bg-transparent !shadow-none !border-transparent hover:!bg-white hover:!shadow-sm hover:!border-gray-100'
+          ? '!bg-transparent !shadow-none hover:!bg-white hover:!shadow-sm'
           : '',
         { 'menu-open': navTheme === 'light' && activeMega !== null },
       ]"
@@ -545,7 +545,6 @@ watch(
 #header[data-nav-theme='light'].menu-open {
   background-color: #ffffff !important;
   box-shadow: 0 1px 2px 0 rgb(0 0 0 / 0.05) !important;
-  border-bottom-color: rgb(229 231 235) !important;
 }
 #header[data-nav-theme='light'].menu-open::before {
   opacity: 1;