{# Finite State theme toggle button — include where you want the sun/moon
control to appear (typically in the top bar / header).
Requires: _theme_init.html in (sets the initial data-theme)
_design_system.html in (provides .fs-theme-toggle styles)
The button shows the icon for the OTHER theme — sun in dark mode (click to
go light), moon in light mode (click to go dark). Persists choice in
localStorage under "fs-theme". #}