feat: improved unit support

Here are two major improvements that could be made to units support, as it occurs to me:

- [ ] Allow for setting a global default unit in the settings, between:
- cm
- em
- in
- in
- mm
- pc
- pt
- px
- rem
- [ ] Respect any manually-set units when using mouse-dragging to reset sizing in the visual editor. In other words, if the width, height, margin, or padding are set to something other than pixels, resizing any of these attributes in the canvas should retain and accordingly resize the attributes while respecting the unit override, instead of reverting to the default unit (which is pixels at present).

No files, tests, or entry points are named. Start by locating the settings flow and the visual-editor mouse-drag resizing behavior described in the issue, then trace how units are currently selected and retained. Done means a configurable default unit works and resizing width, height, margin, and padding preserves manually selected non-pixel units.

Written by the indexing model from the issue text.