Add "Mirror" option for stairs (Cut Back Stairs) #901

Problem or motivation

Stairs can be rotated but not mirrored, so there's no way to get the opposite-handed version of a Cut Back staircase. I can't match my real floorplan, which has the turn going the other way.

Proposed solution

A "Mirror" toggle in the stairs settings panel that flips the stair's handedness in place and keeps all other properties (dimensions, step count, riser height).

Alternatives considered

No response

Additional context

Left- and right-handed stair variants are common in real layouts, and mirroring is a natural companion to the existing rotation control.