fix: prevent white flash when opening mask editor (#9860)

## Summary

- Remove hardcoded `bg-white` from mask editor canvas background div to
prevent white flash on dialog open
- Add a loading spinner while the mask editor initializes (image
loading, canvas setup, GPU resources)
- Background color is set dynamically by `setCanvasBackground()` after
initialization

Fixes #9852


### AS IS


https://github.com/user-attachments/assets/7da61e32-671b-4056-b5ec-8cb246fc7689



### TO BE 

https://github.com/user-attachments/assets/bfdedc69-f690-42c5-8591-619623c04f55



┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9860-fix-prevent-white-flash-when-opening-mask-editor-3226d73d365081de9b7ad4622438e6ed)
by [Unito](https://www.unito.io)

---------

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'