feat(pay): drop-in widget UI + coin-op arcade demo

mountCheckout renders a complete checkout into any container: pay button
with phase-aware labels, progress line next to the button, wallet-popup
announcements, the pre-sign honesty panel, and the receipt. The demo
arcade is a merchant selling game credits through the widget; verified
end-to-end in-browser on MockWallet (7 phases, receipt, credit grant,
game unlock). dist/ is tracked so GitHub Pages serves the demo.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>