Fix layout issues for diff file tree header and fork info

What does this MR do and why?

With new UI enabled, the diff file header is mis-positioned on commit details page. This MR fixes that by adding a new top value to position the header to the correct container.

Additionally this MR adds space in between the actions button in fork info block on narrow screens.

References

Screenshots or screen recordings

	Before	After
Commits details	CleanShot 2025-09-29 at 16.07.57.mp4	CleanShot 2025-09-29 at 15.57.45.mp4
Wiki diff		CleanShot 2025-09-29 at 16.02.09.mp4
Compare diff	CleanShot 2025-09-29 at 16.08.28.mp4	CleanShot 2025-09-29 at 15.58.18-converted.mp4
Fork Info

How to set up and validate locally

• Enable new panel layout
• Go to commit details page. It's better to use a commit that changes multiple files, like this
• Scroll down to make sure the header is sticking to the correct spot

MR acceptance checklist

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