fix(frontend): remove blank gap above nav on non-home pages

Complete the fixed-nav contract: when body.nav-fixed and the header
is not the hero overlay (show-slide), take .header out of flow so
body padding alone reserves space. Avoids double offset from sticky
nav still occupying document flow on single/archive pages.

assets/css/frontend/nav/base.css

@@ -3,6 +3,24 @@ body.nav-fixed {
   padding-top: var(--nav-height, 60px);
 }
 
+/* Fixed nav without hero overlay: take header out of flow.
+   body.nav-fixed already reserves --nav-height via padding-top. */
+body.nav-fixed .header:not(.show-slide) {
+  position: fixed;
+  top: 0;
+  left: 0;
+  right: 0;
+  width: 100%;
+  z-index: 1000;
+}
+
+/* Nav lives inside the fixed header — don't keep sticky (double offset). */
+body.nav-fixed .header:not(.show-slide) .site-nav,
+body.nav-fixed .header:not(.show-slide) .navbar.navbar-top {
+  position: relative;
+  top: auto;
+}
+
 /* Hero sits under transparent fixed/absolute header (zibll slide-header) */
 body.nav-fixed .header-slider-container,
 body.nav-fixed .header-components-container,