fix(dashboard): support Fill Screen for landscape and stabilize its scale

Landscape profiles always sized the window to the full profile/monitor
height, so a partial panel row left a large empty gap and the Fill
Screen toggle was hard-disabled there. Column/scale selection also
maximized zoom against the live window width, so short "-top" profiles
rendered much larger than their transposed portrait counterpart.

Fill Screen now works for landscape too (off content-fits to the rows
shown, on stretches to fill the monitor with panels centered at their
natural scale). Column/row/scale selection is shared between the
renderer and the window-size estimate via one tested function
(landscape_grid_layout) instead of two hand-copied loops, so the two
can no longer drift apart.

Closes #159