Don't stack PR tab menu on small screens (#25789)

the stacking takes up screen space - display the tabs as the navigation
bar. github uses the same layout.

Screenshots (left before, right after):


![image](https://github.com/go-gitea/gitea/assets/1135157/d7e2aaec-c67b-403d-8d56-d4c824b04eed)
![image](https://github.com/go-gitea/gitea/assets/1135157/9e150881-c265-4074-afd7-407bb52e1934)

Large screen:


![image](https://github.com/go-gitea/gitea/assets/1135157/d5cbdaa3-2962-4c4f-9595-5938981ff99e)

templates/repo/pulls/tab_menu.tmpl

@@ -1,4 +1,5 @@
-<div class="ui top attached pull tabular stackable menu">
+<div class="ui pull tabs container">
+	<div class="ui top attached pull tabular menu">
 		<a class="item {{if .PageIsPullConversation}}active{{end}}" href="{{.Issue.Link}}">
 			{{svg "octicon-comment-discussion"}}
 			{{$.locale.Tr "repo.pulls.tab_conversation"}}
@@ -20,4 +21,6 @@
 				<div class="diff-stats-add-bar" style="width: {{Eval 100 "*" .Diff.TotalAddition "/" "(" .Diff.TotalAddition "+" .Diff.TotalDeletion "+" 0.0 ")"}}%"></div>
 			</span>
 		</span>
+	</div>
+	<div class="ui tabs divider"></div>
 </div>

web_src/css/repo.css

@@ -747,8 +747,15 @@
   right: 1px;
 }
 
+.repository.view.issue .pull.tabs.container {
+  width: 100%;
+  max-width: 100%;
+}
+
 .repository.view.issue .pull.tabular.menu {
-  margin-bottom: 1rem;
+  margin-bottom: 0;
+  overflow-x: auto;
+  overflow-y: hidden;
 }
 
 .repository.view.issue .pull.tabular.menu .svg {