feat(ui): regional prompting followups 2

[psychedelicious]

Summary

• Fix subpixel alignment issues
• Add tool hotkeys
• Add image mask to tensor node thanks @JPPhoto
• Revise internal state to support mask layers without text prompts, image masks (like from user uploads)
• Cleanup of rendering logic
• Add rectangle tool - it draws it directly to the mask layer like a paintbucket fill, not a separate layer
• Revise handling for layer color - each layer can set its own transparency, OR you can use the global slider to sync opacity across all mask layers at once
• Update IAIColorPicker to have single-letter labels, which makes this component a bit more compact

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)