Fixes Portable Text editor data issue and contrast for comments

This release fixes critical bugs in the Studio's appearance settings and prevents accidental deletion of entire fields in the Portable Text Input.

• Fixes parts of the Studio, including the comments UI, that followed the OS color scheme instead of the appearance selected in the Studio user menu. The Studio now pins the document to its resolved appearance, including when the hosting application's bundler has down-leveled light-dark().
• Fixes a bug where the Portable Text Input could delete the entire field's content: removing a single block during rapid editing could be escalated to an unset of the whole field, discarding the other blocks.