Manages Mixpanel credentials for the mixpanel_headless library and the mp CLI — checks the active session, lists, adds, and switches accounts, runs OAuth login (one-shot `mp login` or the two-step flow), switches projects and workspaces, and manages saved targets. Use when Mixpanel credentials are missing or failing, when code raises AuthenticationError or reports no account or no project, when the user wants to log in, switch account, project, or workspace, or save or use a target; on 401 or "unauthorized" errors; for "which project am I on?"; or for a login in the EU or India region. Do not use for installing or upgrading the library (use setup) or for analytics questions once credentials work (use mixpanelyst).