Heat oven to 450 degrees F. Place one oven rack in lowest position and a second oven rack in highest position. Lightly coat 2 baking sheets with nonstick cooking spray.
In medium-size bowl, stir together cucumber, yogurt, lemon juice and salt. Cover; refrigerate yogurt sauce.
Place fries on one of the prepared baking sheets. Lightly coat with cooking spray; toss fries with 1 tablespoon lemon-herb seasoning.
Bake fries on lowest rack in 450 degree F oven for 15 minutes.
Meanwhile, season chicken with remaining 2 tablespoons seasoning. Place on second baking sheet.
Place chicken on highest rack in 450 degree F oven; leave fries in oven. Bake until chicken is 170 degrees F, about 15 minutes (fries bake 30 minutes total).
Divide chicken, fries, salad and pita quarters among 6 plates. Sprinkle with feta. Serve yogurt sauce on side.