feat: add cross-parent drag-and-drop with validation feedback

Enable moving nodes between different parents in the tree editor.
Drop targets show blue indicator for valid drops and red pulsing
glow for invalid drops (e.g., dropping onto solution nodes or
onto descendants of the dragged node).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/store/treeEditorStore.ts

@@ -17,8 +17,8 @@ const DRAFT_STORAGE_KEY = 'tree-editor-draft'
 // Helper to generate unique IDs
 const generateId = () => crypto.randomUUID()
 
-// Helper to find a node in the tree structure
-const findNodeInTree = (
+// Helper to find a node in the tree structure (exported for drag validation)
+export const findNodeInTree = (
   nodeId: string,
   structure: TreeStructure | null
 ): TreeStructure | null => {
@@ -144,6 +144,7 @@ interface TreeEditorState {
 
   // Actions - Node ordering
   reorderNodes: (parentId: string, fromIndex: number, toIndex: number) => void
+  moveNode: (nodeId: string, targetParentId: string, targetIndex: number) => void
   reorderOptions: (nodeId: string, fromIndex: number, toIndex: number) => void
 
   // Actions - Selection
@@ -496,6 +497,37 @@ export const useTreeEditorStore = create<TreeEditorState>()(
         get().autoSaveDraft()
       },
 
+      moveNode: (nodeId: string, targetParentId: string, targetIndex: number) => {
+        set((state) => {
+          // Find and remove from current parent
+          const currentParent = findParentNode(nodeId, state.treeStructure)
+          if (!currentParent?.children) return
+
+          const sourceIndex = currentParent.children.findIndex(c => c.id === nodeId)
+          if (sourceIndex === -1) return
+
+          const [movedNode] = currentParent.children.splice(sourceIndex, 1)
+
+          // Find target parent and insert
+          const targetParent = findNodeInTree(targetParentId, state.treeStructure)
+          if (!targetParent) return
+
+          if (!targetParent.children) {
+            targetParent.children = []
+          }
+
+          // Adjust index if moving within same parent and source was before target
+          let adjustedIndex = targetIndex
+          if (currentParent.id === targetParent.id && sourceIndex < targetIndex) {
+            adjustedIndex = targetIndex - 1
+          }
+
+          targetParent.children.splice(adjustedIndex, 0, movedNode)
+          state.isDirty = true
+        })
+        get().autoSaveDraft()
+      },
+
       reorderOptions: (nodeId: string, fromIndex: number, toIndex: number) => {
         set((state) => {
           const node = findNodeInTree(nodeId, state.treeStructure)