[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/explore.css

@@ -1,10 +1,8 @@
-.explore .navbar {
-  margin-bottom: 15px !important;
-  background-color: var(--color-header-wrapper) !important;
+.explore .secondary-nav {
   border-width: 1px !important;
 }
 
-.explore .navbar .svg {
+.explore .secondary-nav .svg {
   width: 16px;
   text-align: center;
   margin-right: 5px;