Do not exceed display for the PR page buttons on smaller screens (#29418)

Fixes #29189.

This is the result after the fix at a width of 768 pixels.

![圖片](https://github.com/go-gitea/gitea/assets/30816317/626d06b3-fd5b-4392-84e1-1191c965aff5)

web_src/css/repo.css

@@ -942,7 +942,7 @@
   margin-bottom: -0.25rem;
 }
 
-@media (max-width: 767.98px) {
+@media (max-width: 991.98px) {
   .repository.view.issue .comment-list .comment .merge-section .item-section {
     align-items: flex-start;
     flex-direction: column;