fix: eliminate visual shaking when adjusting crop region (#8896)

## Summary
Replace the dual-image approach (dimmed <img> + background-image in crop
box) with a single <img> and a box-shadow overlay on the crop box. The
previous approach required two independently positioned images to be
pixel-perfectly aligned, which caused visible jitter from sub-pixel
rounding differences, especially when zoomed in.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8896-fix-eliminate-visual-shaking-when-adjusting-crop-region-3086d73d365081309703d3ab0d4ad44d)
by [Unito](https://www.unito.io)

src/composables/useImageCrop.ts

@@ -243,17 +243,6 @@ export function useImageCrop(nodeId: NodeId, options: UseImageCropOptions) {
     height: `${cropHeight.value * scaleFactor.value}px`
   }))
 
-  const cropImageStyle = computed(() => {
-    if (!imageUrl.value) return {}
-
-    return {
-      backgroundImage: `url(${imageUrl.value})`,
-      backgroundSize: `${displayedWidth.value}px ${displayedHeight.value}px`,
-      backgroundPosition: `-${cropX.value * scaleFactor.value}px -${cropY.value * scaleFactor.value}px`,
-      backgroundRepeat: 'no-repeat'
-    }
-  })
-
   interface ResizeHandle {
     direction: ResizeDirection
     class: string
@@ -605,7 +594,6 @@ export function useImageCrop(nodeId: NodeId, options: UseImageCropOptions) {
     isLockEnabled,
 
     cropBoxStyle,
-    cropImageStyle,
     resizeHandles,
 
     handleImageLoad,