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>

tests/e2e/utils_e2e.js

@@ -52,7 +52,7 @@ export async function save_visual(page) {
       fullPage: true,
       timeout: 20000,
       mask: [
-        page.locator('.dashboard-navbar span>img.ui.avatar'),
+        page.locator('.secondary-nav span>img.ui.avatar'),
         page.locator('.ui.dropdown.jump.item span>img.ui.avatar'),
       ],
     });