Add an ability to drag & drop images over subgraphs with Load Image nodes

### Feature Idea

Hi,

I've started using subgraphs a lot recently. They are very convenient for keeping workflows clean and orderly.

However, I was disappointed to find out that you can't drag and drop images over a Load Image node that resides in a subgraph even though "$$canvas-image-preview" is promoted. You must first enter the subgraph, drag and drop your image over the "Load Image" node, and then exit the subgraph. These are unnecessary steps that should ideally be avoided by making it possible to drop images directly over the subgraph.

### Existing Solutions

_No response_

### Other

Windows 10 Pro, ComfyUI portable v0.6.0, frontend v1.35.9.