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/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>