[Feature request] Dual-panel layout for editing Subgraphs

### 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?

Subgraphs are a powerful feature, but currently you can’t display the main graph and a Subgraph at the same time.
Having to switch between them frequently can significantly hurt the user experience.

One possible way to improve this would be to introduce a dual-panel layout.
To explore the idea, I made a simple prototype extension for ComfyUI.
Instead of replacing the main canvas, this extension opens a right-hand panel that displays the Subgraph.

https://github.com/user-attachments/assets/1f885f6e-3062-442b-aae6-6b17db987fd3

GitHub: [ComfyUI-DualPanel-Subgraph-Viewer](https://github.com/nomadoor/ComfyUI-DualPanel-Subgraph-Viewer)

This is only a proof of concept.
It’s not stable or practical for real use, but it gives a sense of what seamless interaction between the main workflow and a Subgraph could look like.

If such a feature were to be implemented officially, it might be better to design it in a more general and flexible way.
Something similar to Blender’s split-window system could work well, allowing multiple panels to be divided and assigned to different views such as Subgraph, Preview, or Timeline.

I wanted to share this idea and hear what others think.
Many thanks to everyone contributing to the development of ComfyUI.

### When does this problem occur?

When editing Subgraphs in complex workflows.

### How often do you encounter this problem?

Rarely

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

Minor annoyance

### Current workarounds

_No response_

### Ideas for solutions (Optional)

_No response_

### Additional context

_No response_

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-6169-Feature-request-Dual-panel-layout-for-editing-Subgraphs-2926d73d36508114abe3f4909288097d) by [Unito](https://www.unito.io)

Start by reviewing the linked ComfyUI-DualPanel-Subgraph-Viewer prototype and the existing Subgraph editing experience. The issue does not identify implementation files, tests, or a settled design; clarify the panel model and define acceptance criteria before estimating the work.

Written by the indexing model from the issue text.