Refactor frontend tw-justify-between layouts to flex-left-right (#37291)

This PR standardizes left/right two-child frontend layouts on
`flex-left-right` and removes ad-hoc `tw-justify-between` combinations.
The goal is consistent wrapping + spacing behavior under narrow widths
with less utility-class churn.

Also: remove useless "flex-center-wrap", slightly improve some templates
(no visual change, tested)

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: wxiaoguang <2114189+wxiaoguang@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/repo/wiki/new.tmpl

@@ -3,7 +3,7 @@
 	{{template "repo/header" .}}
 	<div class="ui container">
 		{{template "base/alert" .}}
-		<div class="ui header flex-text-block tw-justify-between">
+		<div class="ui header flex-left-right">
 			{{ctx.Locale.Tr "repo.wiki.new_page"}}
 			{{if .PageIsWikiEdit}}
 				<a class="ui tiny primary button" href="{{.RepoLink}}/wiki?action=_new">{{ctx.Locale.Tr "repo.wiki.new_page_button"}}</a>

templates/repo/wiki/pages.tmpl

@@ -2,7 +2,7 @@
 <div role="main" aria-label="{{.Title}}" class="page-content repository wiki pages">
 	{{template "repo/header" .}}
 	<div class="ui container">
-		<h2 class="ui header tw-flex tw-items-center tw-justify-between">
+		<h2 class="ui header flex-left-right">
 			<span>{{ctx.Locale.Tr "repo.wiki.pages"}}</span>
 			<span>
 				{{if and .CanWriteWiki (not .Repository.IsMirror)}}

templates/repo/wiki/revision.tmpl

@@ -3,7 +3,7 @@
 	{{template "repo/header" .}}
 	{{$title := .title}}
 	<div class="ui container">
-		<div class="ui dividing header flex-text-block tw-flex-wrap tw-justify-between">
+		<div class="ui dividing header flex-left-right">
 			<div class="flex-text-block">
 				<a class="ui basic button tw-px-3" title="{{ctx.Locale.Tr "repo.wiki.back_to_wiki"}}" href="{{.RepoLink}}/wiki/{{.PageURL}}">{{svg "octicon-home"}}</a>
 				<div class="tw-flex-1 gt-ellipsis">