The ornamental design for a foldableseat, as shown and described.
1. Foldable seat
1.1 is a left side perspective view of the foldable seat in its folded state; 1.2 is a right side perspective view of the foldable seat in its folded state; 1.3 is a perspective view of the foldable seat in its unfolded state; 1.4 is a top view of the foldable seat in its unfolded state; the bottom view is omitted because it is the same as the top view.