[Feature Request] Support physical units (cm, inches) and DPI/Resolution in "New canvas"

[safvanedge]

Problem / Limitation

Currently, the "New canvas" dialog only accepts raw pixel values (px) and has no concept of resolution (DPI / PPI). If a user attempts to enter a physical dimension like 10cm or 5in, it triggers the error:

"Enter whole numbers from 1 to 30,000 pixels."

For print workflows and graphic design, working with physical dimensions (inches, cm, mm) and print resolutions (e.g., 300 DPI vs 72 PPI for web) is essential.

Proposed Solution

1. Unit Selector: Allow users to select units (px, in, cm, mm) next to Width and Height.
2. Resolution (DPI) Field: Add a Resolution input (defaulting to 72 or 300 pixels/inch) that computes the appropriate pixel dimensions behind the scenes.
3. Smart Input Parsing: If a user types 10cm or 4in directly into the dimension field, automatically recognize the unit and convert it to pixels based on the active resolution rather than rejecting it as an invalid integer.

Context

In NewCanvasSheet.swift, dimensions are currently restricted to static px and validated via CanvasDocument.validDimension as direct integers. Adding physical unit conversion and DPI metadata would significantly improve the workflow for print designers.

[safvanedge]

I'd be happy to submit a PR to fix this if you'd like!

[songhj0728]

In my fork, you can configure PPI settings, choose from various units (mm, px) and formats (A4, B2, etc.), and lock the aspect ratio when creating a new canvas! It would be great if you could try it out and include it in the original!