feat: unified drag-and-drop for skill file tree

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').

web/app/components/workflow/skill/utils/drag-utils.ts

@@ -1,5 +1,27 @@
 import type * as React from 'react'
+import { INTERNAL_NODE_DRAG_TYPE } from '../constants'
 
+// Check if dragging external files from OS
 export const isFileDrag = (e: React.DragEvent): boolean => {
   return e.dataTransfer.types.includes('Files')
 }
+
+// Check if dragging internal tree node
+export const isNodeDrag = (e: React.DragEvent): boolean => {
+  return e.dataTransfer.types.includes(INTERNAL_NODE_DRAG_TYPE)
+}
+
+// Check if any supported drag type
+export const isDragEvent = (e: React.DragEvent): boolean => {
+  return isFileDrag(e) || isNodeDrag(e)
+}
+
+// Get drag action type for tooltip display
+export type DragActionType = 'upload' | 'move'
+export const getDragActionType = (e: React.DragEvent): DragActionType | null => {
+  if (isFileDrag(e))
+    return 'upload'
+  if (isNodeDrag(e))
+    return 'move'
+  return null
+}

web/app/components/workflow/skill/utils/tree-utils.ts

@@ -117,6 +117,19 @@ export function getAllDescendantFileIds(
   return fileIds
 }
 
+export function isDescendantOf(
+  potentialDescendantId: string | null | undefined,
+  ancestorId: string | null | undefined,
+  nodes: AppAssetTreeView[],
+): boolean {
+  if (!potentialDescendantId || !ancestorId)
+    return false
+  if (potentialDescendantId === ancestorId)
+    return true
+  const ancestors = getAncestorIds(potentialDescendantId, nodes)
+  return ancestors.includes(ancestorId)
+}
+
 export function getTargetFolderIdFromSelection(
   selectedId: string | null,
   nodes: AppAssetTreeView[],