Improves all-day event rendering and placement

Simplifies and improves the all-day event rendering process, ensuring
consistent container creation and proper placement of events.

- Ensures all-day containers are consistently created during header
  rendering, preventing potential issues with event placement.
- Removes the complex and unreliable mouseover detection for all-day
  conversion, simplifying the event dragging logic.
- Eliminates the dynamic all-day row height calculation, relying on
  CSS for layout control.
- Prevents errors when the all-day container is missing.

src/renderers/HeaderRenderer.ts

@@ -39,10 +39,10 @@ export abstract class BaseHeaderRenderer implements HeaderRenderer {
   }
 
   /**
-   * Ensure all-day containers exist - but don't create them initially, use lazy creation
+   * Ensure all-day containers exist - always create them during header rendering
    */
   ensureAllDayContainers(calendarHeader: HTMLElement): void {
-    // Do nothing initially - containers will be created when first needed
+    this.createAllDayMainStructure(calendarHeader);
   }
 
   private animateHeaderExpansion(calendarHeader: HTMLElement): void {
@@ -163,6 +163,9 @@ export class DateHeaderRenderer extends BaseHeaderRenderer {
       
       calendarHeader.appendChild(header);
     });
+    
+    // Always create all-day container after rendering headers
+    this.ensureAllDayContainers(calendarHeader);
   }
 }
 
@@ -192,5 +195,7 @@ export class ResourceHeaderRenderer extends BaseHeaderRenderer {
       calendarHeader.appendChild(header);
     });
 
+    // Always create all-day container after rendering headers
+    this.ensureAllDayContainers(calendarHeader);
   }
 }