fixed the dropdown menu for the top New button to expand to the left (#31273) (#31275)

Backport #31273 by @kerwin612

before: 

![1717660314025](https://github.com/go-gitea/gitea/assets/3371163/17ae7a48-31c5-4c71-b285-f65d9106bf86)

after: 

![1717660674763](https://github.com/go-gitea/gitea/assets/3371163/85f847ac-a044-4695-9004-26e6485288c6)

Co-authored-by: Kerwin Bryant <kerwin612@qq.com>

Author: GiteaBot

templates/base/head_navbar.tmpl

@@ -104,7 +104,7 @@
 					<span class="not-mobile">{{svg "octicon-triangle-down"}}</span>
 					<span class="only-mobile">{{ctx.Locale.Tr "create_new"}}</span>
 				</span>
-				<div class="menu">
+				<div class="menu left">
 					<a class="item" href="{{AppSubUrl}}/repo/create">
 						{{svg "octicon-plus"}} {{ctx.Locale.Tr "new_repo"}}
 					</a>