feat: add node picker dropdown to action node form for cross-references

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

frontend/src/store/treeEditorStore.ts

@@ -123,6 +123,25 @@ export const findNodeInTree = (
   return null
 }
 
+/** Collect all nodes in the tree as a flat list with depth info. */
+export function collectAllNodesFlat(
+  root: TreeStructure | null
+): Array<{ id: string; label: string; type: string; depth: number }> {
+  if (!root) return []
+  const result: Array<{ id: string; label: string; type: string; depth: number }> = []
+
+  function walk(node: TreeStructure, depth: number) {
+    const label = node.type === 'decision'
+      ? (node.question || 'Untitled Decision')
+      : (node.title || `Untitled ${node.type}`)
+    result.push({ id: node.id, label, type: node.type, depth })
+    node.children?.forEach(child => walk(child, depth + 1))
+  }
+
+  walk(root, 0)
+  return result
+}
+
 // Helper to find parent of a node
 const findParentNode = (
   nodeId: string,