Extract cell rendering, right-aligned group-icon-strip painting, the
drop-indicator JTree subclass, and drag-and-drop handling out of
ServerTreePanel into ServerTreeCellRenderer, DropIndicatorTree, and
ServerTreeDragAndDrop, leaving ServerTreePanel as the coordinator
(706 -> 284 lines). Pure refactor, no behavior change.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
