Subgraphs can no longer show live previews

### Prerequisites

- [x] I am running the latest version of ComfyUI
- [ ] I have custom nodes enabled

### What happened?

In LG mode, live previews from sampler nodes can no longer be shown on the subgraph.

This used to work in 1.39, but is broken in all newer versions, including the latest release.

In nodes 2.0 it works, but I don't want to use it because the UX is still significantly worse compared to the old design (in particular, labels take too much space away from interactable elements and the display of important values and I unfortunately do not have the CSS skills to fix that since it is not a trivial layout change.)

### Steps to Reproduce

1. Take the default workflow, and put the KSampler inside a subgraph.
2. Run the workflow, and observe no preview.

Unlike previously, "Edit subgraph widgets" does not provide any way to promote the preview widget

### How is this affecting you?

Feature doesn't work as expected

### ComfyUI Frontend Version

1.41.18

### Browser

Firefox

### Console Errors

```javascript

```

### Logs

```shell

```

### Additional Context

_No response_

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-9859-Subgraphs-can-no-longer-show-live-previews-3226d73d3650813197f7fea7c685598e) by [Unito](https://www.unito.io)