I just wanted to chime in (maybe a little late) and agree, agree, agree. Your DD's milestones are eerily similar to those of my DS, who is in other ways a more obviously presenting PGlet. Don't let the well-roundedness throw you off. Although it's pretty common for there to be an area of strength, there are several kids who are above grade-level across the board. It's not a way to exclude giftedness. If anything (and please do read Dr. Ruf's book because she talks about this) it's a sign of profound giftedness.
Coming here and reading more about gifted issues will help you figure out how you can best help your daughter. But, regardless of whether or not she is gifted, or at what level, it sounds like you need to figure out something to improve her school situation. The art show and working on the app sound like great projects and I think you instinctively know what to do to help her. Now, the school admins, that's where you have to plan and strategize.
Oh, and the stumbling on "easy" problems is pretty classic too. My husband said that he was convinced he was stupid because he thought there must be more to the questions because they seemed so easy and yet everyone else thought they were hard. He figured he must be missing something. And my DS often tries to change worksheets or make them more interesting. Last year he brought home a math sheet with the answers written out (nineteen instead of 19).
Good luck and let us know what you find out!