Fix sticky header: remove position:relative override on .container-header

.container-header had position:relative which overrode .sticky-top's
position:sticky due to CSS source order (container-header appeared
later). Removed the conflicting rule. Added .container-header.sticky-top
with z-index:1020 to keep header above all content. Mobile media query
now only disables sticky when the sticky class is present.

Fixes #115

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/media/css/template.css

@@ -14204,18 +14204,23 @@ fieldset>* {
 }
 
 .container-header {
-	position: relative;
 	z-index: 100;
 	background: var(--header-background-image, url('../../../../../../media/templates/site/mokocassiopeia/images/bg.svg'));
 	background-size: var(--header-background-size, auto);
-	-webkit-box-shadow: 0 5px 5px hsla(0, 0%, 0%, 0.03) inset;
 	box-shadow: 0 5px 5px hsla(0, 0%, 0%, 0.03) inset;
 	background-repeat: var(--header-background-repeat, repeat);
 }
 
+/* Sticky header: override z-index to stay above all content */
+.container-header.sticky-top {
+	z-index: 1020;
+}
+
+/* Disable sticky on mobile — full-width header works better as static */
 @media (max-width: 991.98px) {
-	.container-header {
-		position: relative ;
+	.container-header.sticky-top {
+		position: relative;
+		z-index: 100;
 	}
 }