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.

src/managers/EventManager.ts

@@ -30,7 +30,7 @@ export class EventManager {
 
     constructor(eventBus: IEventBus) {
         this.eventBus = eventBus;
-        const timezone = calendarConfig.getTimezone?.() || 'Europe/Copenhagen';
+        const timezone = calendarConfig.getTimezone?.();
         this.dateService = new DateService(timezone);
     }
 
@@ -156,20 +156,23 @@ export class EventManager {
             return null;
         }
 
-        try {
-            if (isNaN(event.start.getTime())) {
-                console.warn(`EventManager: Invalid event start date for event ${id}:`, event.start);
-                return null;
-            }
-
-            return {
-                event,
-                eventDate: event.start
-            };
-        } catch (error) {
-            console.warn(`EventManager: Failed to parse event date for event ${id}:`, error);
+        // Validate event dates
+        const validation = this.dateService.validateDate(event.start);
+        if (!validation.valid) {
+            console.warn(`EventManager: Invalid event start date for event ${id}:`, validation.error);
             return null;
         }
+
+        // Validate date range
+        if (!this.dateService.isValidRange(event.start, event.end)) {
+            console.warn(`EventManager: Invalid date range for event ${id}: start must be before end`);
+            return null;
+        }
+
+        return {
+            event,
+            eventDate: event.start
+        };
     }
 
     /**