fix: toolbar menu being obscured after overflowing (#5930)

#### What type of PR is this?

/kind bug
/area editor
/milestone 2.16.x

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

当工具栏菜单出现溢出时，由于 `justify-content` 会始终保持居中状态，因此会导致溢出的内容被遮挡。

考虑到 [justify-content:safe](https://developer.mozilla.org/zh-CN/docs/Web/CSS/justify-content#safe) 在 safari 上具有兼容性问题，因此修改 HTML 结构，使用 `text-align` 来使菜单居中。

#### How to test it?

测试在菜单栏溢出时，溢出的内容是否被遮挡。

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

Fixes #5926 

#### Does this PR introduce a user-facing change?
```release-note
解决默认编辑器中顶部工具栏菜单溢出后被遮挡的问题
```

ui/packages/editor/src/components/EditorHeader.vue

@@ -59,9 +59,9 @@ function getToolboxItemsFromExtensions() {
 </script>
 <template>
   <div
-    class="editor-header flex items-center py-1 space-x-1 justify-start px-1 overflow-auto sm:!justify-center border-b drop-shadow-sm bg-white"
+    class="editor-header py-1 space-x-1 px-1 overflow-auto border-b drop-shadow-sm bg-white text-center"
   >
-    <div class="inline-flex items-center justify-center">
+    <div class="h-full inline-flex items-center">
       <VMenu>
         <button class="p-1.5 rounded-md hover:bg-gray-100" tabindex="-1">
           <MdiPlusCircle class="text-[#4CCBA0]" />
@@ -80,12 +80,10 @@ function getToolboxItemsFromExtensions() {
           </div>
         </template>
       </VMenu>
-    </div>
       <div class="h-5 bg-gray-100 w-[1px] !mx-1"></div>
       <div
         v-for="(item, index) in getToolbarItemsFromExtensions()"
         :key="index"
-      class="inline-flex items-center justify-center"
       >
         <component
           :is="item.component"
@@ -123,4 +121,5 @@ function getToolboxItemsFromExtensions() {
         </template>
       </div>
     </div>
+  </div>
 </template>