forcing a draw

Sometimes when my opponent fails to resign when completely lost, like naked K vs K, N, R and 3P, I'll promote all the Ps and then go to stalemate or offer a draw. We aren't playing for the world championship here, after all.

No you should not have resigned. White clearly demonstrated they had no idea of the most elementary of the elementary checkmate positions. See my annotation at the 84th move...
Only resign if a easy forced mate and you have no counter, like a lone king vs. king and two rooks OR you are playing against a very strong player (like engines and titled players) and they are winning by a lot.
But don't forget there's a chance for 50-move rule, repetion, timeout ("timeout vs. insufficient material" if you don't have sufficient material to checkmate) and stalemate, like what happened in your game.