Move file browser to a drawer on mobile in Rapid Diffs

What does this MR do and why?

This MR moves file browser to a drawer in Rapid Diffs for a rapid file navigation on narrow screens. It also shows diff stats and view controls on a narrow screen.

Screenshots or screen recordings

Before	After
Screen_Recording_2025-06-02_at_04.38.53	Screen_Recording_2025-06-02_at_04.36.51

How to set up and validate locally

1. Enable rapid_diffs and rapid_diffs_on_mr_show feature flags
2. Go to any merge request
3. Select 'Changes' tab
4. Add ?rapid_diffs=true to the URL, follow it
5. Switch to a narrow viewport (less than 900 pixels wide)
6. Use button in the top right to open file browser
7. Click on any of the files
8. Observe rapid navigation to the file