[Feature]: Sub Graph Preview Panels

### Is there an existing issue for this?

- [x] I have searched the existing issues and checked the recent builds/commits

### What problem are you experiencing?

With the previous group node system, display panels (such as preview image and ksampler preview) were automatically rendered in the group node. However, with the new subgraph functionality, I can't find any way to replicate that. I found that to be really helpful for giving integrated image preview for groups that handled images or sampling.

### When does this problem occur?

- When working with complex subgraphs that would benefit from image previews
- When working with sampler subgraphs where you want to preview the sampling as it happens.

### How often do you encounter this problem?

Daily

### How much does this problem affect your workflow?

Causes moderate inconvenience

### Current workarounds

Currently, the only way to get around this is to have the previews outside of the subgraph. this works for simple things like images, but for sampling, it requires the sampling step to happen outside of the sub graph, which feels counter-intuitive to the encapsulation philosophy of subgraphs.

### Ideas for solutions (Optional)

Have some method of selecting/linking specific nodes to the output as a preview node. any previews that render on this node will be rendered on the subgraph node.

### Additional context

_No response_

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-5043-Feature-Sub-Graph-Preview-Panels-2516d73d365081cbaed3eccb7de1490c) by [Unito](https://www.unito.io)

The issue does not name files, tests, or an entry point. Begin by locating the subgraph node implementation and the existing group-node preview-panel behavior, then determine how a preview node could expose rendered previews through a subgraph. Done means a defined preview-selection design and working previews for image and sampler subgraphs.

Written by the indexing model from the issue text.