Fix(UI): Canvas numeric brush size

[DustyShoe]

Summary

This PR improves brush/eraser size change behavior.
Previously the size did not update when clicking on arrows in numeric field. For update, active cursor in numeric field and hitting Enter was required.
As an addition added responsiveness to slider when holding stepper button and keyboard up/down keys

Related Issues / Discussions

Fixes: #8760

QA Instructions

Check if brush/eraser size changes as expected on:

• "[" and "]" keys
• Holding up/down buttons next to numeric field
• Holding up/down Keyboard keys when numeric field is active.
• Slider updates on every operation

Merge Plan

Should have no impact. It’s tightly scoped: only ToolWidthPicker.tsx. No changes outside the tool width UI.

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)

[Pfannkuchensack]

Works fine. Did not test the [ and ] Hotkeys because they dont work on my keyboard but used another key for it and that works.

[lstein]

Thanks for fixing this! Tested and working as advertised.