Multiple ExpressionEditor nodes share same preview image filename

[prairiefawkes]

Bug

When using multiple ExpressionEditor nodes in a workflow, all nodes write their preview image to the same hardcoded
filename (fe_edit_preview.png). The last node to execute overwrites the preview for all other nodes, making it
appear as though they all produce the same output.

Repro steps

1. Add two or more ExpressionEditor nodes to a workflow
2. Connect them to the same source image
3. Set different expression values on each (e.g. different aaa, eee, woo values)
4. Run the workflow
5. All ExpressionEditor nodes show the same preview image — from whichever node executed last

Root cause

In nodes.py line 932, the preview filename is hardcoded:

filename = "fe_edit_preview.png"

Every instance saves to this same file, so the last one to execute wins.

Fix

Make the filename unique per node instance:

  filename = f"fe_edit_preview_{id(self)}.png"