Dear Pierre,
In 2D planar magnetostatics, you are correct: the field lines are actually the isolines of the z-component of the magnetic vector potential. Note that for an axisymmetric simulation like the one in your image, the scalar field is a_theta and must be multiplied by 2πr for the contour to correspond to the flux lines. The formulation of the PDE also needs to be slightly adapted.
In the webgui, I know it’s possible to plot a scalar field and adjust the color scale to make the contour lines visible, but we lose the intensity coloring of B that we have with FEMM. Another approach is to plot B directly with vector arrows, but then the field lines are somehow lost. I don’t know how to represent certain features of two different fields on the same figure (like the contour of A with the surface filling of B, which is what FEMM is probably doing); if you are proficient in JavaScript, it might be possible: Webgui - programming and internal features — NGS-Py 6.2.2604-2-g455d4e659 documentation
I am not skilled enough to help you further with the webgui. Using matplotlib or another external library could be a workaround. You can evaluate a given field at each point of the mesh (see Access GridFunction with Coordinates ), then use the visualization of your choice.
Best,
Théodore