Workflow-specific Canvas Background Image

# Workflow-specific Canvas Background Image

ComfyUI already supports a custom Canvas background image, but it is currently global.

**I suggest allowing each workflow to have its **own background image**, saved with the workflow.
When a workflow is opened, its associated background should automatically be restored.**

If no workflow-specific background is set, the global Canvas background should be used as the default.
This would make different workflows much easier to visually identify, especially when working with many workflows.

Start by locating the existing global Canvas background image setting and the workflow save and open handling. Trace how workflow data is persisted and restored, then verify that a workflow-specific image is restored on open while workflows without one fall back to the global background.

Written by the indexing model from the issue text.