fix: re-sync draft from store when canvas card is opened

When a decision node is saved with new options, stub next_node_id values
are written back to the store. But the local draft was initialized once at
mount and never refreshed, so reopening the card gave a stale draft with
empty next_node_ids — causing duplicate stubs on every subsequent save.

Fix: reset draft from the live node whenever isExpanded transitions to true.

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

frontend/src/components/tree-editor/TreeCanvasNode.tsx

@@ -1,4 +1,4 @@
-import { useState, useCallback } from 'react'
+import { useState, useCallback, useEffect } from 'react'
 import {
   HelpCircle,
   Zap,
@@ -97,13 +97,22 @@ export function TreeCanvasNode({
     cloneNodeWithoutChildren(node)
   )
 
-  // Reset draft if node changes while editing (e.g. store update from undo)
+  // Reset draft if node ID changes (e.g. navigating between nodes)
   const [lastNodeId, setLastNodeId] = useState(node.id)
   if (node.id !== lastNodeId) {
     setDraft(cloneNodeWithoutChildren(node))
     setLastNodeId(node.id)
   }
 
+  // Re-sync draft from store whenever the card is opened, so stale next_node_id
+  // values (written back after stub creation) don't cause duplicate stubs on re-save
+  useEffect(() => {
+    if (isExpanded) {
+      setDraft(cloneNodeWithoutChildren(node))
+    }
+  // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [isExpanded])
+
   const handleDraftUpdate = useCallback((updates: Partial<TreeStructure>) => {
     setDraft((prev) => ({ ...prev, ...updates }))
   }, [])