Fix FTB toggle layout shift

What does this MR do and why?

Ensures the stop spacing for the FTB between expanded/collapses states match

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the repository_file_tree_browser feature flag
2. Navigate to a directory or file in the repository
3. Toggle the file tree browser expanded/collapsed states
4. No layout shift should occur on the toggle button

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #567775