First, the container seems to large for that size tree. Second, the foliage is setting in the window where the surface of leaves receive direct sun and they heat up during the day, while the container (trees root system) is setting below the window level where it can't receive direct sun. The container should be raised higher so the sun's rays will then also heat up the root system. The foliage of a citrus tree can wilt from the root system being too dry, and it can also wilt from the root area being too wet. When the medium holds too much water, it displaces the required oxygen from the area of the root system. When a citrus tree's roots are deprived of oxygen, the roots cannot take up water. Oxygen is required for the root system to do any work. I don't know how often you water the tree but it should not water again until approximately the top two inches of the medium feels dry. You can test the moisture lever by sticking you finger into the medium. I don't know if your tree's medium is too dense or not, but with as much perlite as I see mixed in, it could be ok. You can normally tell by how fast the water drains out the bottom after you water it. Are you watering too often? From looking at the leaves they don't seem to be deficient in minerals. Good luck to you and your little tree.