feat(go): M6.6.1 — QR payment popup modal on /adults and /juniors

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>

go/internal/web/static/css/app.css

@@ -442,6 +442,23 @@ tr:hover {
 }
 
 /* QR Modal styles */
+#qrModal {
+    display: none !important;
+    position: fixed;
+    top: 0;
+    left: 0;
+    width: 100vw;
+    height: 100vh;
+    background-color: rgba(0, 0, 0, 0.9);
+    z-index: 9999;
+    justify-content: center;
+    align-items: center;
+}
+
+#qrModal.active {
+    display: flex !important;
+}
+
 #qrModal .modal-content {
     max-width: 400px;
     text-align: center;