feat: header z-index

fixes certain issues with the header appearing below things sometimes

set to 1000 to make sure it appears above everything, unless other elements have a higher value which is unlikely

addon/header-sticky.css

···
 	header {
 		position: sticky;
 		top: 0;
+		z-index: 1000;
 		background-color: var(--dc-bg-2);
 	}
 	@supports (backdrop-filter: blur(24px)) {