applets/taskmanager: Reduce width of items when in horizontal panel

The width of traditional Task Manager items is more than in Plasma 5
and looks odd. The reason is due to porting away from
`PlasmaCore.Theme.mSize(font).width` and using
`Kirigami.Units.iconSizes.sizeForLabels` instead.

This metric is meant to be used for icon size calculations, so instead
let's use `Kirigami.Units.gridUnit` according to the porting guide.

BUG: 476824