MITCHELL — Five years after ordering a cheek swab kit on a whim, Kelsey Miller made a decision that took minutes — and eventually gave someone she’ll never meet a chance at more time. Joining a national stem cell donor registry felt simple at first, almost forgettable. The Mitchell native signed up after seeing a story on the Today Show about a child in need of a match, thinking it was just another small way to help, like donating blood. “I didn…