Align Diff expansion footer buttons

What does this MR do and why?

Closes: #284119 (closed)

Screenshots or screen recordings

Here's how it looks for different highlight themes:

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Old variant of the MR

Before	After

There are some edge cases on very tiny screens, but I think it's good enough for most cases. We can handle these edge cases with Container Queries when they gain better support in browsers.

How to set up and validate locally

1. Open Merge Request Changes tab that contains diff for a file that has too many unchanged lines to show

MR acceptance checklist

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