Replace bare <a href=/qr> Pay buttons with <button data-*> elements that
open an in-page #qrModal (matching Python's showPayQR UX), driven by a
new payment-qr.js vanilla-JS IIFE module. Remove the now-dead qrHref /
qrHrefAll template helpers from render.go.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
