The ornamental design for a frontdoor casing, as shown and described.
FIG. 1 is a front left perspective view of a front door casing according to the present invention.
FIG. 2 is a front right perspective view of the front door casing shown in FIG. 1.
FIG. 3 is a rear left perspective view thereof.
FIG. 4 is a rear right perspective view thereof.
FIG. 5 is a front view thereof.
FIG. 6 is a rear view thereof.
FIG. 7 is a left side view thereof.
FIG. 8 is a right side view thereof.
FIG. 9 is a top view thereof; and,
FIG. 10 is a bottom view thereof.
The broken lines comprise claim boundaries and neither the portions inside the broken lines nor the broken lines themselves form any part of the claimed invention.