Align umbriel tiled windows with a floating bar via struts

In the umbriel compositor, tiled windows sit layout.gap logical px from the usable area edge: edgePad = gap + border_width, so the border's outer edge lands at exactly gap. A floating layer-shell bar (e.g. noctalia) with its own side margin will look misaligned. Fix with layout.struts.left/right set to (bar margin - gap). Measure the difference from a physical-pixel screenshot and divide by the output scale to get logical px. niri accepts the same layout.struts keys.