Improve PR sticky header layout on mobile

### Description

How it's right now:

<img width="536" height="220" alt="Image" src="https://github.com/user-attachments/assets/78826e8a-c651-42a4-96a9-71a8241f4ff3" />

What we can do:

<img width="526" height="221" alt="Image" src="https://github.com/user-attachments/assets/8150d25b-e60b-407f-8c14-40a8af466a04" />


### Example URLs

https://github.com/refined-github/refined-github/pull/9165