flat-twin-shares-geometry-not-material

When a user setting picks between two renderings of the same control — an expensive decorative one and a plain twin — every geometry decision has to be mirrored between them (item width, bar height, indicator size, inset, and the rule that computes the width budget) so toggling changes the material and never the layout; the compiler cannot pair two constants declared in two files. Also covers recolouring an icon…