fix: drop the D-pad center square

Render a clear gap instead of the plain center cell so the D-pad
reads as an open cross.

Co-Authored-By: qwen3.8-27b@q3_k_xl: removed the D-pad center cell

src/app/GamepadViews.swift

@@ -18,8 +18,7 @@ struct DPadView: View {
             cellView(.dPadUp)
             HStack(spacing: gap) {
                 cellView(.dPadLeft)
-                RoundedRectangle(cornerRadius: 6)
-                    .fill(Color.secondary.opacity(0.28))
+                Color.clear
                     .frame(width: cell, height: cell)
                 cellView(.dPadRight)
             }