Improves all-day event drag and drop

Refactors all-day event drag and drop handling for improved accuracy and performance.

Introduces a shared `ColumnDetectionUtils` for consistent column detection.

Simplifies all-day conversion during drag, placing events in row 1 and calculating the column from the target date.

Implements differential updates during drag end, updating only changed events for smoother transitions.

src/utils/ColumnDetectionUtils.ts

@@ -0,0 +1,94 @@
+/**
+ * ColumnDetectionUtils - Shared utility for column detection and caching
+ * Used by both DragDropManager and AllDayManager for consistent column detection
+ */
+
+export interface ColumnBounds {
+  date: string;
+  left: number;
+  right: number;
+}
+
+export class ColumnDetectionUtils {
+  private static columnBoundsCache: ColumnBounds[] = [];
+
+  /**
+   * Update column bounds cache for coordinate-based column detection
+   */
+  public static updateColumnBoundsCache(): void {
+    // Reset cache
+    this.columnBoundsCache = [];
+
+    // Find alle kolonner
+    const columns = document.querySelectorAll('swp-day-column');
+
+    // Cache hver kolonnes x-grænser
+    columns.forEach(column => {
+      const rect = column.getBoundingClientRect();
+      const date = (column as HTMLElement).dataset.date;
+
+      if (date) {
+        this.columnBoundsCache.push({
+          date,
+          left: rect.left,
+          right: rect.right
+        });
+      }
+    });
+
+    // Sorter efter x-position (fra venstre til højre)
+    this.columnBoundsCache.sort((a, b) => a.left - b.left);
+  }
+
+  /**
+   * Get column date from X coordinate using cached bounds
+   */
+  public static getColumnDateFromX(x: number): string | null {
+    // Opdater cache hvis tom
+    if (this.columnBoundsCache.length === 0) {
+      this.updateColumnBoundsCache();
+    }
+
+    // Find den kolonne hvor x-koordinaten er indenfor grænserne
+    const column = this.columnBoundsCache.find(col =>
+      x >= col.left && x <= col.right
+    );
+
+    return column ? column.date : null;
+  }
+
+  /**
+   * Get column index (1-based) from date
+   */
+  public static getColumnIndexFromDate(date: string): number {
+    // Opdater cache hvis tom
+    if (this.columnBoundsCache.length === 0) {
+      this.updateColumnBoundsCache();
+    }
+
+    const columnIndex = this.columnBoundsCache.findIndex(col => col.date === date);
+    return columnIndex >= 0 ? columnIndex + 1 : 1; // 1-based index
+  }
+
+  /**
+   * Get column index from X coordinate
+   */
+  public static getColumnIndexFromX(x: number): number {
+    const date = this.getColumnDateFromX(x);
+    return date ? this.getColumnIndexFromDate(date) : 1;
+  }
+
+  /**
+   * Clear cache (useful for testing or when DOM structure changes)
+   */
+  public static clearCache(): void {
+    this.columnBoundsCache = [];
+  }
+
+  /**
+   * Get current cache for debugging
+   */
+  public static getCache(): ColumnBounds[] {
+    return [...this.columnBoundsCache];
+  }
+}