feat(ui): revised canvas scale snapping

[psychedelicious]

Summary

Revised scale snapping on canvas.

Tracks the "intended" scale separately from "effective" scale to allow for snapping that does not inadvertently lock the scale to snap points, as described in #8048.

Also removed a couple constraints that were causing some jank:

• Canvas Scale is no longer rounded, allowing the scaling on touchpads to be smoother and get "stuck" less
• Canvas Position is no longer rounded, preventing the drift of canvas position while scaling in and out

Related Issues / Discussions

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QA Instructions

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Merge Plan

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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)