The Browns are so desperate for a QB it's possible they could trade for Kap and still take a QB with their first pick to give them some options but I don't see Kap being a good fit in Cleveland not that any QB is. He has too many issues passing from the pocket to have success in Cleveland he would be running around all the time. SF isn't going to give Kap away they're going to want a #2 for him but they could settle for a 3rd to unload him because he doesn't want to be there.
You don't see many teams trade for a QB then turn around and use a top 5 pick on one in the draft. There's one thought that maybe the Browns aren't that thrilled with either Wentz or Goff or that they're tired of wasting first round picks on QBs. The last 2 QBs they drafted in the first round ended up on the street after only 2 seasons. They've had no luck with drafted QBs or veteran QBs would hate to be a fan of the Browns. Look at the list of QBs they've gone through.
https://en.wikipedia.org/wiki/List_of_Cleveland_Browns_starting_quarterbacks