fix(frontend): stop single sidebar float overlap under fixed nav

Replace the fragile content-wrap float + negative-margin sidebar with a
main.container grid, add fixed-nav top breathing room so the breadcrumb
stays below the header, and mark the aside as content-layout__aside.

assets/css/frontend/nav/base.css

@@ -21,6 +21,16 @@ body.nav-fixed .header:not(.show-slide) .navbar.navbar-top {
   top: auto;
 }
 
+/*
+ * Non-home pages have no hero under the fixed header. Keep the first content
+ * (breadcrumb / article) clearly below the bar even if a fluid widget zone is
+ * empty or a child uses negative margin.
+ */
+body.nav-fixed:not(:has(.header.show-slide)) .fluid-widget-wrap:first-of-type:empty + main.container,
+body.nav-fixed:not(:has(.header.show-slide)) main.container:first-of-type {
+  scroll-margin-top: calc(var(--nav-height, 60px) + 8px);
+}
+
 /* Hero sits under transparent fixed/absolute header (zibll slide-header) */
 body.nav-fixed .header-slider-container,
 body.nav-fixed .header-components-container,