Fix color and roundness for file tree browser file item

What does this MR do and why?

Fix color and roundness for file tree browser file item

• Update to use gl-action-neutral-colors for hover state
• Update to use gl-highlight-target-xx for selected state
• Update the item to have lg border radius
• Bold file name for selected item
• Align indentation line to the file row container
• Remove size prop on file icon since the number is invalid

References

• Update the selected element to have a border-radius of 8px. #567775 (comment 2851589520)
• Add hover color to a selectable line item to design token button.default.tertiary.background.color.hover (light gray), and change the color of the selected element to button.confirm.tertiary.background.color.hover (light blue). #567775 (comment 2851589520)

Screenshots or screen recordings

CleanShot 2025-11-27 at 12.28.37.mp4

How to set up and validate locally

• Enable repository_file_tree_browser feature flag
• Go to the repository view, click on any folders/files (note, this won't show on project overview page)
• Expand the file tree browser with toggle to the left of ref selector in the header
• Observe hover and selected styling

MR acceptance checklist

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