Enhance container.SortZone for Complex Layouts (reopened from #7207)

Originally: #7207

Goal

To refactor and enhance Neo.draggable.container.SortZone to support more complex drag-and-drop scenarios, such as dragging a component via a child handle. This will make the base class more generic, powerful, and reusable, significantly reducing the need for complex overrides in subclasses like DashboardSortZone.

Tasks

1. Add dragHandleSelector Config:

   • In src/draggable/container/SortZone.mjs, introduce a new configuration property: dragHandleSelector.
   • Type: String
   • Default: null
   • Purpose: This selector will identify a child element within a sortable item that serves as its drag handle.

2. Refactor onDragStart:

   • Modify the onDragStart method to incorporate the new dragHandleSelector.
   • If dragHandleSelector is set, the logic must:
     • Verify the drag was initiated on an element matching the selector.
     • Identify the parent component that owns the handle as the actual item to be dragged.
     • Implicitly filter the owner's items to only consider components containing a dragHandleSelector element as sortable. All internal logic (e.g., creating itemRects) should operate on this filtered list.
   • If dragHandleSelector is null, the method must fall back to its original behavior to ensure backward compatibility.

3. Ensure Method Compatibility:

   • Review and adjust other methods (onDragEnd, switchItems, updateItem) to ensure they correctly handle the filtered list of sortable items and the associated index mappings.

Acceptance Criteria

• When dragHandleSelector is configured on a SortZone, dragging an element matching the selector initiates a drag operation for its parent component.
• Items within the SortZone's owner that do not contain an element matching the dragHandleSelector are not considered sortable and are ignored by the drag-drop logic.
• The DashboardSortZone can be refactored into a minimal subclass that primarily just configures the dragHandleSelector.
• Existing functionalities that use container.SortZone (e.g., tab header reordering) remain unaffected and work as before.