fix(ui): give the segment control a visible selected state

[Renekakpo]

Why:

• Selection was signalled by a soft shadow over two near-identical backgrounds: 1.06:1 contrast in the light theme, 1.27:1 in dark. There was effectively nothing to see, and nothing at all for a screen reader, since no accessibility helper in the codebase could emit accessibilityState.

What:

• The selected segment now takes the accent fill the rest of the app already uses for selection, reaching 9.12:1 and 4.54:1. The sliding indicator is gone: it never rendered under test, so selected state had never been asserted anywhere. State now lives on each button, which makes it announceable and testable, and removed the only left-to-right assumptions in the control.