Function start_window_drag 

pub fn start_window_drag(
    state: &Arc<AppState>,
    args: &Value,
) -> Result<Value, String>

Initiate window drag (for frameless windows). Windows: sends WM_NCLBUTTONDOWN/HTCAPTION via Win32 — find_own_top_level_window resolves the per-process HWND so multi-window works without a label. Linux/macOS: dispatches CefWindow::BeginWindowDrag on the UI thread; needs the source window’s label so non-main windows drag themselves rather than the main window. Frontend reads ?windowLabel=… from its URL and passes it here; missing → “main” for backward compatibility.