One reply at a time, and no more "data changed" errors

Opening the same file twice, or sending a message again while the first was still being answered, could end in a "data changed by another user" error or run the same request twice. Opening a file no longer competes with anything, and a chat or an in-app assistant session now answers one message at a time: a second send while a reply is running is told so, or follows the reply already under way.