Introduce .secondary-nav and handle .page-content spacing universally (#29982)

Fixes: https://github.com/go-gitea/gitea/issues/29981. Introduce
`.secondary-nav` as a universal way for styling and margin adjustments
inside `.page-content`.

If the first child of `.page-content` is `.secondary-nav`, we add margin
below it, otherwise we add padding to the first child. Notable changes:

- `--color-header-wrapper` is replaced with `--color-secondary-nav-bg`.
- `navbar` class is removed.

---------

Co-authored-by: Giteabot <teabot@gitea.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/admin/layout_head.tmpl

@@ -1,6 +1,6 @@
 {{template "base/head" .ctxData}}
 <div role="main" aria-label="{{.ctxData.Title}}" class="page-content {{.pageClass}}">
-	<div class="ui container gt-mb-4">
+	<div class="ui container">
 		{{template "base/alert" .ctxData}}
 	</div>
 	<div class="ui container fluid padded flex-container">

templates/explore/navbar.tmpl

@@ -1,4 +1,4 @@
-<overflow-menu class="ui secondary pointing tabular top attached borderless menu navbar">
+<overflow-menu class="ui secondary pointing tabular top attached borderless menu secondary-nav">
 	<div class="overflow-menu-items tw-justify-center">
 		<a class="{{if .PageIsExploreRepositories}}active {{end}}item" href="{{AppSubUrl}}/explore/repos">
 			{{svg "octicon-repo"}} {{ctx.Locale.Tr "explore.repos"}}

templates/repo/header.tmpl

@@ -1,4 +1,4 @@
-<div class="header-wrapper">
+<div class="secondary-nav">
 {{with .Repository}}
 	<div class="ui container">
 		<div class="repo-header">
@@ -128,7 +128,7 @@
 		{{if .IsGenerated}}<div class="fork-flag">{{ctx.Locale.Tr "repo.generated_from"}} <a href="{{(.TemplateRepo ctx).Link}}">{{(.TemplateRepo ctx).FullName}}</a></div>{{end}}
 	</div>
 {{end}}
-	<overflow-menu class="ui container secondary pointing tabular top attached borderless menu navbar tw-pt-0 tw-my-0">
+	<overflow-menu class="ui container secondary pointing tabular top attached borderless menu tw-pt-0 tw-my-0">
 		{{if not (or .Repository.IsBeingCreated .Repository.IsBroken)}}
 			<div class="overflow-menu-items">
 				{{if .Permission.CanRead $.UnitTypeCode}}

templates/user/auth/link_account.tmpl

@@ -1,6 +1,6 @@
 {{template "base/head" .}}
 <div role="main" aria-label="{{.Title}}" class="page-content user link-account">
-	<overflow-menu class="ui secondary pointing tabular top attached borderless menu navbar tw-bg-header-wrapper">
+	<overflow-menu class="ui secondary pointing tabular top attached borderless menu secondary-nav">
 		<div class="overflow-menu-items tw-justify-center">
 			<!-- TODO handle .ShowRegistrationButton once other login bugs are fixed -->
 			{{if not .AllowOnlyInternalRegistration}}

templates/user/auth/signin_navbar.tmpl

@@ -1,5 +1,5 @@
 {{if or .EnableOpenIDSignIn .EnableSSPI}}
-<overflow-menu class="ui secondary pointing tabular top attached borderless menu navbar tw-bg-header-wrapper">
+<overflow-menu class="ui secondary pointing tabular top attached borderless menu navbar secondary-nav">
 	<div class="overflow-menu-items tw-justify-center">
 		<a class="{{if .PageIsLogin}}active {{end}}item" rel="nofollow" href="{{AppSubUrl}}/user/login">
 			{{ctx.Locale.Tr "auth.login_userpass"}}

templates/user/auth/signup_openid_navbar.tmpl

@@ -1,4 +1,4 @@
-<overflow-menu class="ui secondary pointing tabular top attached borderless menu navbar tw-bg-header-wrapper">
+<overflow-menu class="ui secondary pointing tabular top attached borderless menu secondary-nav">
 	<div class="overflow-menu-items tw-justify-center">
 		<a class="{{if .PageIsOpenIDConnect}}active {{end}}item" href="{{AppSubUrl}}/user/openid/connect">
 			{{ctx.Locale.Tr "auth.openid_connect_title"}}

templates/user/dashboard/navbar.tmpl

@@ -1,4 +1,4 @@
-<div class="dashboard-navbar">
+<div class="secondary-nav tw-border-b tw-border-b-secondary">
 	<div class="ui secondary stackable menu">
 		<div class="item">
 			<div class="ui floating dropdown jump">
@@ -105,4 +105,3 @@
 	{{end}}
 	</div>
 </div>
-<div class="divider tw-mt-0"></div>