After exiting a Subgraph, the exited subgraph should be visibly highlighted

### Feature Idea

Working with repeating sets of subgraphs, it is hard to remember which subgraph you've just worked in after exiting. If the exited Subgraph were to be visibly highlighted, this wouldn't be a problem.

### Existing Solutions

_No response_

### Other

_No response_

Start by locating the code that handles exiting a subgraph and the graph-selection or highlighting behavior in the ComfyUI interface. Trace how the active subgraph changes when exiting, then verify that the exited subgraph is visibly highlighted afterward and that the behavior works with repeating sets of subgraphs.

Written by the indexing model from the issue text.