The hero's send button matches the nav

The hero's send button takes the nav button's colour. In dark mode the
prompt box's send pill was the brand periwinkle while "Open Assembly" sat a few
hundred pixels above it in white: two differently treated primaries in one view.
Light had them both near-black already, which is why it took a dark-mode
screenshot to notice. Both fills now name the nav's own, so the pair matches on
background, text colour and radius in either theme.

Light reads var(--color-neutral-900) rather than #171717 — that is what the
nav actually renders, and a shade darker than the hex that had been sitting
there. They looked the same and were not.

The "Unlock more features" popover carries the same label a few pixels away and
deliberately shared the old accent, so it follows the caller's fill rather than
being left as the one periwinkle button on the page.

Not yet auth-aware, deliberately: the button still reads "Get started" to a
signed-in visitor, but it points at signup, so relabelling it without moving the
destination would promise "Build it" and deliver a signup page. Where a
logged-in person should land is a product question. Tracked in WEB3-7.

Also in this release, from main:

• The homepage "Businesses powered" figure goes from 1,000 to 1,250 (WEB3-5).
• The accounting client portal hero reads "The accounting client portal you can
  build on" in place of "…your clients won't ignore".
• The pull request template is rewritten around Summary / Linear / Testing.