Coordinate grid uses same scale as user's units (#7465)

Closes https://github.com/KittyCAD/engine/issues/3494. Thanks to @nadr0 for helping on the JS side.

If users set their units, the grid will stop auto scaling, and instead will be set to 10 of whatever unit they used. 

If users set their units, and those units are metric, then it'll include a scale bar (see screenshot). Imperial units won't have that bar. 

This behaviour is configurable via settings.

## Limitations

 - The scale bar below the grid cannot be disabled in metric units, and cannot be enabled in imperial units

<img width="1690" alt="Screenshot 2025-06-05 at 7 51 41 PM" src="https://github.com/user-attachments/assets/c597087c-f96d-4c30-95f4-b3d8ba2b5567" />

docs/kcl-lang/settings/user.md

@@ -85,6 +85,13 @@ Whether to show the debug panel, which lets you see various states of the app to
 
 **Default:** None
 
+##### fixed_size_grid
+
+If true, the grid cells will be fixed-size, where the width is the user's default length unit. If false, the grid's size will scale as the user zooms in and out.
+
+
+**Default:** true
+
 
 #### modeling