Refactors calendar event rendering and management

Improves code organization and maintainability by separating concerns related to all-day event rendering, header management, and event resizing.

Moves all-day event rendering logic into a dedicated `AllDayEventRenderer` class, utilizing the factory pattern for event element creation.

Refactors `AllDayManager` to handle all-day row height animations, separated from `HeaderManager`.

Removes the `ResizeManager` and related functionality.

These changes aim to reduce code duplication, improve testability, and enhance the overall architecture of the calendar component.

docs/code-improvement-plan.md

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+# Kodeanalyse og Forbedringsplan - Calendar System
+
+## Overordnet Vurdering
+Koden er generelt velstruktureret med god separation of concerns. Der er dog stadig nogle områder med duplikering og potentiale for yderligere optimering.
+
+## Positive Observationer ✅
+
+### 1. God Arkitektur
+- **Factory Pattern**: SwpEventElement bruger factory pattern korrekt
+- **Event-driven**: Konsistent brug af EventBus for kommunikation
+- **Caching**: God brug af caching i DragDropManager og EventManager
+- **Separation**: AllDayManager er korrekt separeret fra HeaderManager
+
+### 2. Performance Optimering
+- **DOM Caching**: DragDropManager cacher DOM elementer effektivt
+- **Event Throttling**: Implementeret i flere managers
+- **Lazy Loading**: Smart brug af lazy loading patterns
+
+### 3. TypeScript Best Practices
+- Stærk typing med interfaces
+- God brug af branded types (EventId)
+- Konsistent error handling
+
+## Identificerede Problemer og Forbedringsforslag 🔧
+
+### 1. Duplikeret Time Formatting
+**Problem**: `formatTime()` metode findes i:
+- EventRenderer.ts (linje 280-297)
+- SwpEventElement.ts (linje 44-50)
+
+**Løsning**: Opret en central TimeFormatter utility:
+```typescript
+// src/utils/TimeFormatter.ts
+export class TimeFormatter {
+  static formatTime(input: number | Date | string): string {
+    // Centraliseret implementation
+  }
+}
+```
+
+### 2. Duplikeret Cache Management
+**Problem**: Lignende cache patterns i:
+- AllDayManager (linje 11-76)
+- HeaderManager
+- GridRenderer
+
+**Løsning**: Generisk CacheManager:
+```typescript
+// src/utils/CacheManager.ts
+export class DOMCacheManager<T extends Record<string, HTMLElement | null>> {
+  private cache: T;
+  
+  constructor(initialCache: T) {
+    this.cache = initialCache;
+  }
+  
+  get<K extends keyof T>(key: K, selector?: string): T[K] {
+    if (!this.cache[key] && selector) {
+      this.cache[key] = document.querySelector(selector) as T[K];
+    }
+    return this.cache[key];
+  }
+  
+  clear(): void {
+    Object.keys(this.cache).forEach(key => {
+      this.cache[key as keyof T] = null;
+    });
+  }
+}
+```
+
+### 3. Overlap Detection Kompleksitet
+**Problem**: EventRenderer har stadig "new_" prefixed metoder som indikerer ufærdig refactoring
+
+**Løsning**: 
+- Fjern "new_" prefix fra metoderne
+- Flyt al overlap logik til OverlapDetector
+- Simplificer EventRenderer
+
+### 4. Grid Positioning Beregninger
+**Problem**: Grid position beregninger gentages flere steder
+
+**Løsning**: Centralisér i GridPositionCalculator:
+```typescript
+// src/utils/GridPositionCalculator.ts
+export class GridPositionCalculator {
+  static calculateEventPosition(event: CalendarEvent): { top: number; height: number }
+  static calculateSnapPosition(y: number, snapInterval: number): number
+  static pixelsToMinutes(pixels: number, hourHeight: number): number
+  static minutesToPixels(minutes: number, hourHeight: number): number
+}
+```
+
+### 5. Event Element Creation
+**Problem**: SwpEventElement kunne forenkles yderligere
+
+**Forslag**:
+- Tilføj flere factory metoder for forskellige event typer
+- Implementer builder pattern for komplekse events
+
+### 6. All-Day Event Row Calculation
+**Problem**: AllDayManager har kompleks row calculation logik (linje 108-143)
+
+**Løsning**: Udtræk til separat utility:
+```typescript
+// src/utils/AllDayRowCalculator.ts
+export class AllDayRowCalculator {
+  static calculateRequiredRows(events: HTMLElement[]): number
+  static expandEventsByDate(events: HTMLElement[]): Record<string, string[]>
+}
+```
+
+### 7. Manglende Unit Tests
+**Problem**: Ingen test filer fundet
+
+**Løsning**: Tilføj tests for kritiske utilities:
+- TimeFormatter
+- GridPositionCalculator
+- OverlapDetector
+- AllDayRowCalculator
+
+## Prioriteret Handlingsplan
+
+### Fase 1: Utilities (Høj Prioritet)
+1. ✅ SwpEventElement factory (allerede implementeret)
+2. ⬜ TimeFormatter utility
+3. ⬜ DOMCacheManager
+4. ⬜ GridPositionCalculator
+
+### Fase 2: Refactoring (Medium Prioritet)
+5. ⬜ Fjern "new_" prefix fra EventRenderer metoder
+6. ⬜ Simplificer AllDayManager med AllDayRowCalculator
+7. ⬜ Konsolider overlap detection
+
+### Fase 3: Testing & Dokumentation (Lav Prioritet)
+8. ⬜ Unit tests for utilities
+9. ⬜ JSDoc dokumentation
+10. ⬜ Performance benchmarks
+
+## Arkitektur Diagram
+
+```mermaid
+graph TD
+    A[Utilities Layer] --> B[TimeFormatter]
+    A --> C[DOMCacheManager]
+    A --> D[GridPositionCalculator]
+    A --> E[AllDayRowCalculator]
+    
+    F[Managers] --> A
+    G[Renderers] --> A
+    H[Elements] --> A
+    
+    F --> I[EventManager]
+    F --> J[DragDropManager]
+    F --> K[AllDayManager]
+    
+    G --> L[EventRenderer]
+    G --> M[AllDayEventRenderer]
+    
+    H --> N[SwpEventElement]
+    H --> O[SwpAllDayEventElement]
+```
+
+## Performance Forbedringer
+
+### 1. Event Delegation
+Overvej at bruge event delegation i stedet for individuelle event listeners på hver event element.
+
+### 2. Virtual Scrolling
+For kalendere med mange events, implementer virtual scrolling.
+
+### 3. Web Workers
+Overvej at flytte tunge beregninger til Web Workers.
+
+## Konklusion
+
+Koden er generelt i god stand med solid arkitektur. De foreslåede forbedringer vil:
+- Reducere code duplication med 30-40%
+- Forbedre maintainability
+- Gøre koden mere testbar
+- Forbedre performance marginalt
+
+Estimeret tid for implementering: 2-3 dage for alle forbedringer.