server backend is a stub

backend_mode, api_url, and api_token all exist in the config, and selecting backend_mode: "server" renders a "server mode coming soon!" cat screen, but there is no API integration behind it at all.

Long-term this is the plan — dew eventually talks to https://todo.myn.sh/api — but it is explicitly on the backburner for now. Filing it so the dead config keys and the placeholder screen don't read as accidental in the meantime. Either implement it or drop the keys until it's real.