fix: use actual root node ID in orphan validation check

AI-generated trees use descriptive IDs (e.g., "verify-account-exists")
instead of "root", causing the root node to be falsely flagged as orphaned.

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

frontend/src/store/treeEditorStore.ts

@@ -855,7 +855,7 @@ export const useTreeEditorStore = create<TreeEditorState>()(
 
         // Check for orphaned nodes (not root and not referenced)
         allNodeIds.forEach(id => {
-          if (id !== 'root' && !referencedIds.has(id)) {
+          if (id !== state.treeStructure?.id && !referencedIds.has(id)) {
             // Check if it's a direct child of another node (via children array)
             let isChildOfAny = false
             const checkIfChild = (node: TreeStructure) => {