In-TUI MCP OAuth login modal.
MCPLoginScreen is both a Textual ModalScreen and an implementation of
OAuthInteraction. The login worker awaits its interaction methods while
the user sees and acts on the modal's widgets — authorize URLs become
clickable links, paste-back callback URLs go through an inline input row,
device-code instructions render inline, and the modal closes itself on
success.
The screen runs on the Textual event loop (same loop as the worker), so
methods called from the worker can await modal-bound futures directly
without app.call_from_thread.
