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;

go/internal/web/static/js/payment-qr.js

@@ -0,0 +1,60 @@
+(function () {
+  'use strict';
+
+  const container = document.getElementById('filterContainer');
+  if (!container) return;
+  const bankAccount = container.dataset.bankAccount || '';
+
+  const modal     = document.getElementById('qrModal');
+  const titleEl   = document.getElementById('qrTitle');
+  const imgEl     = document.getElementById('qrImg');
+  const accountEl = document.getElementById('qrAccount');
+  const amountEl  = document.getElementById('qrAmount');
+  const messageEl = document.getElementById('qrMessage');
+  if (!modal || !titleEl || !imgEl || !accountEl || !amountEl || !messageEl) return;
+
+  // Convert "YYYY-MM" → "MM/YYYY"; "+"-joined ranges converted piece-wise.
+  // Mirrors templates/adults.html showPayQR logic.
+  function toCzechMonth(rawMonth) {
+    return rawMonth.split('+')
+      .map(function (p) { return p.replace(/^(\d{4})-(\d{2})$/, '$2/$1'); })
+      .join('+');
+  }
+
+  function showQR(name, amount, month, rawMonth) {
+    var numericMonth = toCzechMonth(rawMonth);
+    var message = name + ': ' + numericMonth;
+
+    titleEl.innerText   = 'Payment for ' + month;
+    accountEl.innerText = bankAccount;
+    amountEl.innerText  = amount;
+    messageEl.innerText = message;
+
+    imgEl.src = '/qr?'
+      + 'account=' + encodeURIComponent(bankAccount)
+      + '&amount=' + encodeURIComponent(amount)
+      + '&message=' + encodeURIComponent(message);
+
+    modal.classList.add('active');
+  }
+
+  function closeQR() {
+    modal.classList.remove('active');
+  }
+
+  document.addEventListener('click', function (ev) {
+    var btn = ev.target.closest('.pay-btn');
+    if (!btn) return;
+    ev.preventDefault();
+    showQR(
+      btn.dataset.name,
+      btn.dataset.amount,
+      btn.dataset.month,
+      btn.dataset.rawMonth
+    );
+  });
+
+  document.addEventListener('keydown', function (e) {
+    if (e.key === 'Escape' && modal.classList.contains('active')) closeQR();
+  });
+}());