feat(ui): canvas stage, zoom, and fit improvements

[psychedelicious]

Summary

• Fix issue where creating a new canvas from an image using one of the pictured buttons can result in the new layer being offset from the bbox, if the bbox had previously been moved.

• Fix issue where canvas isn't always centered and zoomed correctly when creating new canvas from an image
• Add a spinner that renders when the canvas is busy rasterizing, compositing (the pause between clicking Invoke and the graph getting queued) or calculating the bbox rect. Just a subtle hint that "we are doing something right now, don't be alarmed that nothing is happening"
• Add quick animation when fitting canvas view to make it less jarring

Related Issues / Discussions

Reported somewhere in discord. I think the two issues have existed for a long time and are not new.

QA Instructions

Demo of new behaviours:

Screen.Recording.2025-06-06.at.2.01.06.pm.mov

Note:

• Spinner in bottom right corner while we are calculating the bbox (same spinner shows during other operatiosn as described above)
• Things are centered and zoomed correctly when creating new layer
• Tweened position and scale when fitting

Merge Plan

n/a

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)