Improves date handling and event stacking

Enhances date validation and timezone handling using DateService, ensuring data integrity and consistency.

Refactors event rendering and dragging to correctly handle date transformations.

Adds a test plan for event stacking and z-index management.

Fixes edge cases in navigation and date calculations for week/year boundaries and DST transitions.

wwwroot/css/calendar-events-css.css

@@ -59,7 +59,6 @@ swp-day-columns swp-event {
     opacity: 0.8;
     left: 2px;
     right: 2px;
-    margin-left: 0px;
     width: auto;
   }