Bug: Offset while using brush tool on iPad

### Description

- Large offset while using the web version on iPad.
- Using the apple pencil to draw with the brush tool on the iPad surface, there is a large noticable offset of the ink appearing from the tip of the pencil touching the screen.
- The brush ink also flows outside of the artboard (although I suspect this might be by desgin)

### Video Demonstration
https://drive.google.com/file/d/1buYIvX035Be9K-pn0Rn7eCGvYZ4jJTX1/view?usp=sharing

### Other details

- Safari version: 26
- iOS version: iPadOS 26.0
- iPad model: iPad Pro 13 inch, 2024

### Preliminary thoughts

- The mismatch is CSS px vs device px and/or origin differences.
look at https://github.com/GraphiteEditor/Graphite/blob/master/editor/src/messages/tool/tool_messages/brush_tool.rs

Start with editor/src/messages/tool/tool_messages/brush_tool.rs and reproduce the offset using the web version on the reported iPad and Apple Pencil setup. Compare the pointer position with the rendered brush location, considering the reported CSS-pixel, device-pixel, and origin differences. Done means brush strokes follow the pencil tip without the reported offset and the observed artboard behavior is verified.

Written by the indexing model from the issue text.