feat(editor): add idle status for embed iframe block (#11142)

To close:
[BS-2843](https://linear.app/affine-design/issue/BS-2843/iframe-embed-block-占位态)
[BS-2844](https://linear.app/affine-design/issue/BS-2844/iframe-embed-block-create-modal-ui-调整)
[BS-2880](https://linear.app/affine-design/issue/BS-2880/spotify-选中时圆角有问题)
[BS-2881](https://linear.app/affine-design/issue/BS-2881/miro-圆角有问题-点击-see-the-board-加载之后就好了)

blocksuite/affine/blocks/block-embed/src/embed-iframe-block/embed-edgeless-iframe-block.ts

@@ -14,7 +14,10 @@ export class EmbedEdgelessIframeBlockComponent extends toGfxBlockComponent(
 
   override blockDraggable = false;
 
-  override accessor blockContainerStyles = { margin: '0' };
+  override accessor blockContainerStyles = {
+    margin: '0',
+    backgroundColor: 'transparent',
+  };
 
   get edgelessSlots() {
     return this.std.get(EdgelessLegacySlotIdentifier);