Prevent flash of dropdown menu on labels list (#30215) (#30216)

Backport #30215 by @silverwind

On the labels list, This `left` class caused the dropdown content to
flash on page load until JS had hidden it. Remove it as I see no purpose
to it.

<img width="215" alt="image"
src="https://github.com/go-gitea/gitea/assets/115237/9e1de97f-dd89-41e0-9229-5c4a786ba762">

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/repo/issue/labels/label_list.tmpl

@@ -8,7 +8,7 @@
 					{{ctx.Locale.Tr "repo.issues.filter_sort"}}
 				</span>
 				{{svg "octicon-triangle-down" 14 "dropdown icon"}}
-				<div class="left menu">
+				<div class="menu">
 					<a class="{{if or (eq .SortType "alphabetically") (not .SortType)}}active {{end}}item" href="?sort=alphabetically&state={{$.State}}">{{ctx.Locale.Tr "repo.issues.label.filter_sort.alphabetically"}}</a>
 					<a class="{{if eq .SortType "reversealphabetically"}}active {{end}}item" href="?sort=reversealphabetically&state={{$.State}}">{{ctx.Locale.Tr "repo.issues.label.filter_sort.reverse_alphabetically"}}</a>
 					<a class="{{if eq .SortType "leastissues"}}active {{end}}item" href="?sort=leastissues&state={{$.State}}">{{ctx.Locale.Tr "repo.milestones.filter_sort.least_issues"}}</a>

web_src/css/modules/header.css

@@ -135,6 +135,12 @@ h4.ui.header .sub.header {
   font-weight: var(--font-weight-normal);
 }
 
+/* open dropdown menus to the left in right-attached headers */
+.ui.attached.header > .ui.right .ui.dropdown .menu {
+  right: 0;
+  left: auto;
+}
+
 /* if a .top.attached.header is followed by a .segment, add some margin */
 .ui.segments + .ui.top.attached.header,
 .ui.attached.segment + .ui.top.attached.header {