Fixes drag and drop to header issues

Addresses two key issues related to dragging events to the header: the premature removal of the original event element and the creation of duplicate all-day events.

The original event is no longer removed when dragging to the header; it is now only removed upon a successful drop.

Also, it prevents the creation of duplicate all-day events by checking for existing all-day events before creating new ones, using DOM queries to ensure accurate state.

docs/drag-drop-header-complete-bug-analysis.md

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+# Komplet Bug Analyse: Drag-Drop Header Problemer
+
+## Identificerede Problemer
+
+### Problem 1: Original Element Fjernes For Tidligt
+**Lokation**: AllDayManager.handleConvertToAllDay() linje 274
+```typescript
+originalElement.remove();  // ❌ FORKERT - skal ikke fjernes her
+```
+
+### Problem 2: Duplicate All-Day Events ved Mouseover
+**Lokation**: AllDayManager.handleConvertToAllDay() linje 232-285
+
+Hver gang `header:mouseover` fyrer (hvilket kan ske mange gange under drag), oprettes et NYT all-day event uden at checke om det allerede eksisterer.
+
+## Event Flow Problem
+
+```mermaid
+sequenceDiagram
+    participant Mouse
+    participant Header
+    participant AllDay as AllDayManager
+    
+    Note over Mouse: Dragger over header
+    loop Hver mouseover event
+        Mouse->>Header: mouseover
+        Header->>AllDay: drag:convert-to-allday
+        AllDay->>AllDay: Opretter NYT all-day event ❌
+        Note over AllDay: Ingen check for eksisterende!
+    end
+    Note over AllDay: Resultat: Multiple all-day events!
+```
+
+## Løsning
+
+### Fix 1: Check for Eksisterende All-Day Event
+```typescript
+// AllDayManager.handleConvertToAllDay()
+private handleConvertToAllDay(targetDate: string, originalElement: HTMLElement): void {
+    const eventId = originalElement.dataset.eventId;
+    
+    // CHECK: Eksisterer all-day event allerede?
+    const existingAllDay = document.querySelector(
+        `swp-allday-container swp-allday-event[data-event-id="${eventId}"]`
+    );
+    
+    if (existingAllDay) {
+        console.log('All-day event already exists, skipping creation');
+        return; // Exit early - don't create duplicate
+    }
+    
+    // Fortsæt med at oprette all-day event...
+}
+```
+
+### Fix 2: Fjern IKKE Original Element
+```typescript
+// Linje 274 - skal fjernes eller kommenteres ud:
+// originalElement.remove();  // <-- FJERN DENNE LINJE
+```
+
+### Fix 3: Track Conversion State (Optional)
+For at undgå gentagne conversions, kunne vi tracke state:
+
+```typescript
+// AllDayManager
+private convertedEventIds = new Set<string>();
+
+private handleConvertToAllDay(targetDate: string, originalElement: HTMLElement): void {
+    const eventId = originalElement.dataset.eventId;
+    
+    // Check if already converted
+    if (this.convertedEventIds.has(eventId)) {
+        return;
+    }
+    
+    // Mark as converted
+    this.convertedEventIds.add(eventId);
+    
+    // Create all-day event...
+}
+
+private handleConvertFromAllDay(draggedEventId: string): void {
+    // Clear conversion state
+    this.convertedEventIds.delete(draggedEventId);
+    
+    // Remove all-day event...
+}
+```
+
+## Komplet Fix Checklist
+
+1. **AllDayManager.handleConvertToAllDay()**
+   - [ ] Tilføj check for eksisterende all-day event
+   - [ ] Fjern/kommenter `originalElement.remove()` linje 274
+   - [ ] Log når duplicate undgås
+
+2. **AllDayManager.handleConvertFromAllDay()**
+   - [ ] Verificer at all-day event faktisk fjernes
+   - [ ] Clear evt. conversion state
+
+3. **Test Scenarios**
+   - [ ] Hurtig mouseover frem og tilbage over header
+   - [ ] Langsom drag over header
+   - [ ] Drag ind i header → ud → ind igen
+   - [ ] Multiple events dragges samtidig (hvis muligt)
+
+## Root Causes
+
+1. **Manglende idempotency**: handleConvertToAllDay() er ikke idempotent
+2. **Forkert element lifecycle**: Original fjernes for tidligt
+3. **Manglende state tracking**: Ingen tracking af hvilke events der er konverteret
+
+## Performance Overvejelser
+
+HeaderManager throttler mouseover events (linje 41-49), men det er ikke nok når musen bevæger sig langsomt over header. Vi skal have idempotent logik.
+
+## Implementeringsrækkefølge
+
+1. **Først**: Fix duplicate all-day events (check for existing)
+2. **Derefter**: Fjern originalElement.remove()
+3. **Test**: Verificer at begge problemer er løst
+4. **Optional**: Implementer conversion state tracking for bedre performance