[Single file editor] Refine border radius on editor
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## Problem to solve
This issue represents a finding from our work to audit features belonging to ~"group::source code". See the [analysis in Figma](https://www.figma.com/design/7UQAtMGbZtEeqiaiBMqah5/Dark-mode-%3E-Source-Code-Group?node-id=19-667&p=f&t=FbNy3i0ybTtzHfDa-0) for more information. See the [overview video](https://youtu.be/0wEmZLcF_CU) for guidance on how to review this analysis.
[Finding purple #1](https://www.figma.com/design/7UQAtMGbZtEeqiaiBMqah5/Dark-mode-%3E-Source-Code-Group?node-id=18-351&t=JV29SHoPpKlCqyJh-4)
## Screenshot

## Proposal
At minimum:
* Refine border radius on editor so that all corners are rounded and respected by nested content.
* Add space between tabs and tab content.
* Additional notes and guidance to resolve this finding:
* Should the tabs have a space between them and the editor? Currently the border radius right up against the tabs is odd and could benefit from being offset.
* The inner element appears to not respect the border radius (both sides).
Proposed update:
- Follow a pattern similar to the comment editor where there are no tabs and the ability to preview is inline with the editor controls.
- When previewing, the control would change to 'Continue editing'.
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