Update subcondition nesting and reorder interactions

[saravanpa-aot]

Summary

• add explicit subcondition indent/outdent controls for changing hierarchy levels
• limit drag-and-drop to reordering within the same parent level
• preserve trailing siblings as children when a subcondition is moved out one level
• update subcondition persistence logic in the API to handle nested tree updates and empty lists correctly
• add backend tests covering nested subcondition updates and removal