refactor: 统一导航栏阴影样式，提升视觉一致性

src/@core/scss/placeholders/_default-layout.scss

@@ -5,13 +5,13 @@
 // Vertical nav scrolled sticky elevated nav
 %default-layout-vertical-nav-scrolled-sticky-elevated-nav {
   background-color: rgb(var(--v-theme-surface));
-  box-shadow: 0 4px 8px -4px rgb(94 86 105 / 42%);
+  box-shadow: var(--app-surface-shadow);
 }
 
 // Floating navbar and sticky elevated navbar scrolled
 %default-layout-vertical-nav-floating-navbar-and-sticky-elevated-navbar-scrolled {
   background-color: rgb(var(--v-theme-surface));
-  box-shadow: 0 4px 8px -4px rgb(94 86 105 / 42%);
+  box-shadow: var(--app-surface-shadow);
 }
 
 // Floating navbar overlay

src/@layouts/components/VerticalNavLayout.vue

@@ -347,7 +347,9 @@ export default defineComponent({
     .layout-wrapper.layout-horizontal-nav-active.layout-horizontal-nav-scrolled.layout-navbar-fixed .layout-navbar {
       backdrop-filter: blur(12px) saturate(1.2);
       background: rgb(var(--v-theme-surface)) !important;
-      box-shadow: 0 4px 8px -4px rgb(94 86 105 / 42%);
+      box-shadow:
+        0 1px 3px rgba(0, 0, 0, 4%),
+        0 1px 2px rgba(0, 0, 0, 2%);
     }
 
     .layout-wrapper.layout-horizontal-nav-active.layout-horizontal-nav-scrolled.layout-navbar-fixed

src/styles/common.scss

@@ -222,7 +222,7 @@ html[data-theme-shadow='high'] {
   }
 }
 
-.v-btn:not(.v-btn--variant-plain, .v-btn--variant-text, .v-btn--flat) {
+.v-btn:not(.v-btn--variant-plain, .v-btn--variant-text, .v-btn--flat, .theme-horizontal-nav__item) {
   box-shadow: var(--app-surface-shadow) !important;
   transition: box-shadow 0.2s ease;
 }