Ten-year-old Zoe Elias has perfect piano dreams. She can practically feel the keys under her flying fingers; she can hear the audience's applause. All she needs is a baby grand so she can start her lessons, and then she'll be well on her way to Carnegie Hall.             But when Dad ventures to the music store and ends up with a wheezy organ instead of a piano, Zoe's dreams hit a sour note. Learning the organ versions of old TV theme songs just isn't the same as mastering Beethoven on the piano. And the organ isn't the only part of Zoe's life that's off-kilter, what with Mom constantly at work, Dad afraid to leave the house, and that odd boy, Wheeler Diggs, following her home from school every day.             Yet when Zoe enters the annual Perform-O-Rama organ competition, she finds that life is full of surprises--and that perfection may be even better when it's just a little off center.