allDay property

Whether this widget is an all-day chip rather than a timed event: false for the timed-event overlay (#78), true for the all-day-chip overlay (#80). A host that wires one builder to both entryBuilder and allDayEntryBuilder branches on this to render a chip vs. a timed card. All-day chips don't sub-divide a column or drag, so their layout always carries columnIndex: 0, columnCount: 1, dragType: DragType.none and isDragged: false; the chip's stacking lane is already baked into size and the widget's on-screen position.