Adjusts week header z-index

Lowers the z-index of the week header to ensure it slides under the header-spacer during horizontal scrolling.

This prevents the week header from overlapping the header-spacer, improving the visual flow of the calendar layout.

wwwroot/css/calendar-layout-css.css

@@ -212,7 +212,7 @@ swp-week-header {
   border-bottom: 1px solid var(--color-border);
   position: sticky;
   top: 0;
-  z-index: 5; /* Higher than time-axis to cover it when scrolling */
+  z-index: 3; /* Lower than header-spacer so it slides under during horizontal scroll */
   height: 80px; /* Fixed height */
 }