The homepage's two send buttons match again

The homepage's second send button matches the first. 2026-08-25.3 changed
the hero composer's submit fill to the nav's colour and missed the closing CTA's,
which is the same control further down the same page — so the top box went white
on dark while the bottom one stayed periwinkle.

Both boxes now name the nav's primary in both themes, and no periwinkle submit
fill is left in the tree. Measured against the nav on the homepage: hero and
closing composer both identical in background and text colour, light and dark.