1 - Solve it by moving it outside. I'd recommend following the principle that all non-private types should be in their own source file.
2 - I'm less clear what the problem is here, even by moving the enum to its own file, I'd stick with your names:
public enum LineShape {Straight, Bend, Whatever};
public class FigureLine
{
public LineShape Shape {get; private set;}
}
LineShape
is a better name than just Shape
as it better describes what the enum is for (defining line styles, rather than eg 2D or 3D shapes).