Feature/brush tool

[kancheng]

This PR focuses only on the brush drawing tool:

• Add brush size control
• Add a cancel/delete action for brush drawing
• Fix a Qt6 crash caused by float arguments passed to QPainter.drawEllipse
• Update brush button positions

No localization or unrelated changes are included.

[kancheng]

demo details

[kancheng]

如需詳細演示，請參閱：#1726

[kancheng]

@wkentaro

I wanted to briefly explain the current CI status. At the moment, the only failing check is:

ci / check (pull_request)

In this PR, the only functional changes I made are in canvas.py (related to the drawing/brush functionality). The other CI jobs (builds on macOS, Ubuntu, and Windows) are all passing successfully, so there are no runtime or cross-platform issues.

I’ve just updated my branch to be fully in sync with the latest upstream state to avoid any drift. Based on my local testing and the successful build checks, the feature itself should be stable and mature at this point.

[kirillmeisser]

Hi, @kancheng thanks for your work! Me and my company would also be very interested in this feature. What is holding it back from being merged? Or do you know why the ci / check job is failing?