fix(ui): Reset canvas session when queue item is canceled

[hipsterusername]

Summary

Fixes the waiting for image screen sticking around after a cancel.

Related Issues / Discussions

N/A

QA Instructions

Cancel an image

Merge Plan

Merge the PR

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

[psychedelicious]

This will reset the whole staging area when a single item is canceled.

I think the right way to solve this is:

• Add a computed in invokeai/frontend/web/src/features/controlLayers/components/SimpleSession/context.tsx. It derives a boolean isPending from the $items atom by iterating over them and checking if any are pending/in progress.
• Add a corresponding boolean flag to CanvasStagingAreaModule.
• Update CanvasStagingAreaModule.connectToSession to accept this new computed. Add it to the effect and sync the state of hte computed with the corresponding flag in CanvasStagingAreaModule.
• Use this flag to determine if we should render the placeholder or nothing at all.