[backport core/1.41] fix: prevent white flash when opening mask editor (#10042)

Backport of #9860 to `core/1.41`

Automatically created by backport workflow.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10042-backport-core-1-41-fix-prevent-white-flash-when-opening-mask-editor-3256d73d365081b1aafac55ec47acced)
by [Unito](https://www.unito.io)

Co-authored-by: Dante <bunggl@naver.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/components/maskeditor/MaskEditorContent.vue

@@ -35,9 +35,11 @@
           'z-40': store.activeLayer === 'mask'
         }"
       />
-      <div ref="canvasBackgroundRef" class="size-full bg-white" />
+      <div ref="canvasBackgroundRef" class="size-full" />
     </div>
 
+    <LoadingOverlay :loading="!initialized" size="sm" />
+
     <div class="maskEditor-ui-container flex min-h-0 flex-1 flex-col">
       <div class="flex min-h-0 flex-1 overflow-hidden">
         <ToolPanel
@@ -77,6 +79,8 @@ import { useDialogStore } from '@/stores/dialogStore'
 import { useMaskEditorDataStore } from '@/stores/maskEditorDataStore'
 import { useMaskEditorStore } from '@/stores/maskEditorStore'
 
+import LoadingOverlay from '@/components/common/LoadingOverlay.vue'
+
 import BrushCursor from './BrushCursor.vue'
 import PointerZone from './PointerZone.vue'
 import SidePanel from './SidePanel.vue'