Function handle_viewer_panes 

async fn handle_viewer_panes(__arg0: State<AppState>) -> Response

GET /agentmux/viewer/panes: every pane on this computer, window by window and tab by tab, with each agent’s shell drawer (backend::viewer::panes): names, types, the agent name a feed opens by, and whether a terminal runs. No content, command line or working directory; agents hidden from paired devices, and their shells, are left out.