Switch from native HTML5 drag to react-arborist's built-in drag system
for internal node drag-and-drop. The HTML5Backend used by react-arborist
was intercepting dragstart events, preventing native drag from working.
- Add onMove callback and disableDrop validation to Tree component
- Sync react-arborist drag state (isDragging, willReceiveDrop) to Zustand
- Simplify use-node-move to only handle API execution
- Update use-unified-drag to only handle external file uploads
- External file drops continue to work via native HTML5 events
Implement unified drag system that supports both internal node moves
and external file uploads with consistent UI feedback. Uses native
HTML5 drag API with shared visual states (isDragOver, isBlinking,
DragActionTooltip showing 'Move to' or 'Upload to').
