Improves all-day event drag and drop

Enhances the drag and drop experience for all-day events by expanding the header to display the all-day row when dragging an event over it.

Introduces constants for all-day event layout.

wwwroot/css/calendar-layout-css.css

@@ -151,6 +151,7 @@ swp-calendar-header {
   top: 0;
   z-index: 3; /* Lower than header-spacer so it slides under during horizontal scroll */
   height: calc(var(--header-height) + var(--all-day-row-height)); /* Same calculation as spacers */
+  transition: height 0.3s ease;
   
   /* Force scrollbar to appear for alignment */
   overflow-y: scroll;