Fix button size in "attached header right" (#28770)
Before:
<details>
![image](https://github.com/go-gitea/gitea/assets/2114189/c82f8615-04d4-408f-95bf-689c227a8814)
![image](https://github.com/go-gitea/gitea/assets/2114189/d01d05a7-09ca-48f3-a0e7-fec48119b426)
</details>
After:
![image](https://github.com/go-gitea/gitea/assets/2114189/e560398e-2fa4-4f46-9229-f6acc70e32ab)
![image](https://github.com/go-gitea/gitea/assets/2114189/bd18961d-0636-4584-b893-c465daaf40af)