The ornamental design for a headboardunit, as shown and described.
FIG. 1 is a right front elevational perspective view of a headboard unit
showing my new design;
FIG. 2 is a top view of the headboard unit; and
FIG. 3 is a bottom view of the headboard unit;
FIG. 4 is a front elevational view of the headboard unit; and,
FIG. 5 is a left side view of the headboard unit, the right view being a
mirror image in appearance, the rear view having having an outline
corresponding to a mirror image of the front view, not having other
ornamental design.