Feat(UI): Add linear and radial gradient tools to canvas

[DustyShoe]

Summary

This PR adds a set of radial and linear gradients to available toolset on canvas.
Adds parent and child custom buttons.
Tool respects color transparency and both color swatches. It draws gradient in direction of mouse movement: from active color to inactive. Gradient is rasterized on mouse release on current active layer.
It is also possible to activate clipping by drag start-stop points.
Gradient is clipped by bbox to save on performance.

2026-01-18.16-30-16.mp4

Related Issues / Discussions

QA Instructions

• Verify buttons display is correct.
• Verify gradient direction follows mouse movement from active color to inactive color.
• Change both color swatches and confirm both are used in the gradient.
• Set opacity on one or both colors and verify transparency is respected.
• Draw gradients of different sizes and confirm rendering is clipped to the bounding box.
• Activate "Clip Gradient" and verify clip by start-end points is applied.
• Use Undo/Redo and verify each gradient application is a single history step.

Merge Plan

PR merge should be painless. Though, code might need a proper check.

Checklist

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

[DustyShoe]

Also would like an advise on what icon should "Gradient Clip" button have.
Right now it uses "Crop" icon: