Rebuild the theme identity control and fix the icon collision (#265)
* Replace the docs theme identity select with a matching pill control
The theme identity picker was a raw native <select>, the only native
form control on the docs surface, squeezed to ~200px in the sidebar
footer and truncating "Tactile". Add a text-label segmented toggle
group alongside the existing icon-only one, sharing the same pill
shape, height, and focus treatment as the colour-mode group beside it,
and use it for theme identity in both the sidebar footer and the
playground header.
Also give system colour mode a settings-gear glyph so it no longer
collides with the monitor icon used for the Desktop viewport option.
* Swap the system colour-mode glyph for a half-filled circle
A settings cog reads as "open theme options", not "follow the
operating system" — a misleading glyph, not just an ambiguous one.
Replace it with the conventional split-circle contrast mark (one
half filled, one half outline), which reads as "either of these two,
decided for you". Matches the existing icon set's outline
conventions (24x24 viewBox, currentColor stroke) apart from the
filled half the mark requires.