Guest signup, mobile responsive, WCAG contrast, and in-app payment
history all verified via tunnel. Payment history's per-row receipt
link requirement accepted as satisfied by the 'Advanced billing in
Helcim' escape hatch (Helcim's card-transactions API doesn't expose
per-row receipt URLs). Also corrects the mobile breakpoint note —
chrome sidebar hides at 768px, in-page columns collapse at 1024px.
