[Port] gitea#29982 Introduce .secondary-nav and handle .page-content spacing universally

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>

---

Conflict resolution: Trivial conflict & changed selector to reflect new
classes.
Ref: https://codeberg.org/forgejo/forgejo/issues/2776
(cherry picked from commit 3ccda41a539b8ba7841919ee12dc2877ddc03818)

web_src/css/modules/navbar.css

@@ -136,3 +136,7 @@
   justify-content: center;
   z-index: 1; /* prevent menu button background from overlaying icon */
 }
+
+.secondary-nav {
+  background: var(--color-secondary-nav-bg) !important; /* important because of .ui.secondary.menu */
+}