[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/dashboard.css

@@ -77,22 +77,22 @@
   margin: 0 1px; /* Accommodate for Semantic's 1px hacks on .attached elements */
 }
 
-.dashboard .dashboard-navbar {
-  padding: 4px 12px;
+.dashboard .secondary-nav {
+  padding: 1px 12px; /* match .overflow-menu-items in height */
 }
 
-.dashboard .dashboard-navbar .right.menu {
+.dashboard .secondary-nav .right.menu {
   gap: .35714286em;
 }
 
-.dashboard .dashboard-navbar .right.menu div.item {
+.dashboard .secondary-nav .right.menu div.item {
   padding-left: 0.5rem;
 }
 
-.dashboard .dashboard-navbar .org-visibility .label {
+.dashboard .secondary-nav .org-visibility .label {
   margin-left: 5px;
 }
 
-.dashboard .dashboard-navbar .ui.dropdown {
+.dashboard .secondary-nav .ui.dropdown {
   max-width: 100%;
 }