perf: improve shadow style of side menu (#3638)

#### What type of PR is this?

/kind improvement
/area console
/milestone 2.4.0

#### What this PR does / why we need it:

优化左侧菜单栏边框阴影的样式。

#### Which issue(s) this PR fixes:

Fixes https://github.com/halo-dev/halo/issues/3629

#### Does this PR introduce a user-facing change?

```release-note
优化左侧菜单栏边框阴影的样式。
```

console/src/layouts/BasicLayout.vue

@@ -405,8 +405,8 @@ onMounted(() => {
 .navbar {
   @apply w-64;
   @apply bg-white;
+  @apply shadow;
   z-index: 999;
-  box-shadow: 0 4px 4px #f6c6ce;
 
   .profile-placeholder {
     height: 70px;