From 919845518c9ad79e909d392b1e314535827640dc Mon Sep 17 00:00:00 2001
From: Jan Novak <kacerr.cz@gmail.com>
Date: Fri, 8 May 2026 14:49:35 +0200
Subject: [PATCH] =?UTF-8?q?feat(go):=20M6.6.1=20=E2=80=94=20QR=20payment?=
 =?UTF-8?q?=20popup=20modal=20on=20/adults=20and=20/juniors?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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>
---
 ...6-05-08-1439-go-m6-6-1-payment-qr-modal.md | 335 ++++++++++++++++++
 go/internal/web/html_handler_test.go          |  46 +++
 go/internal/web/render.go                     |  33 +-
 go/internal/web/static/css/app.css            |  17 +
 go/internal/web/static/js/payment-qr.js       |  60 ++++
 go/internal/web/templates/adults.tmpl         |  24 +-
 go/internal/web/templates/juniors.tmpl        |  24 +-
 7 files changed, 501 insertions(+), 38 deletions(-)
 create mode 100644 docs/plans/2026-05-08-1439-go-m6-6-1-payment-qr-modal.md
 create mode 100644 go/internal/web/static/js/payment-qr.js

diff --git a/docs/plans/2026-05-08-1439-go-m6-6-1-payment-qr-modal.md b/docs/plans/2026-05-08-1439-go-m6-6-1-payment-qr-modal.md
new file mode 100644
index 0000000..f29d69e
--- /dev/null
+++ b/docs/plans/2026-05-08-1439-go-m6-6-1-payment-qr-modal.md
@@ -0,0 +1,335 @@
+# M6.6.1 — Pay-button QR popup modal (`/adults`, `/juniors`)
+
+> Plan-mode note: per project convention this plan should live at
+> `docs/plans/2026-05-08-1439-go-m6-6-1-payment-qr-modal.md`. The first
+> execution step after approval is to copy this file there.
+> Branch: `feat/go-m6-6-1-payment-qr-modal` off `main`.
+
+## Context
+
+M6.6 ([docs/plans/2026-05-08-1334-go-m6-6-action-pages.md](docs/plans/2026-05-08-1334-go-m6-6-action-pages.md))
+landed the backend `/qr` PNG endpoint plus `/sync-bank`, `/flush-cache`,
+`/version`. While doing it, the **client-side** half of the Pay flow was
+collapsed from a modal-with-details into a plain `<a href="/qr?...">` —
+clicking Pay now navigates the whole tab to the raw PNG.
+
+The Python app instead opens an in-page `#qrModal` showing the QR image
+*plus* labelled details (account, amount, message, title), then closes
+the modal on outside-click or `[close]`
+([templates/adults.html:631-648](templates/adults.html#L631-L648),
+JS at [templates/adults.html:963-993](templates/adults.html#L963-L993)).
+This is the UX the user expects from the dashboard — staying on the
+table while displaying the QR is significantly more practical than
+losing context to a fullscreen image.
+
+M6.6.1 restores parity: the Pay/Pay-All buttons open an in-page
+`#qrModal`, identical in structure to the Python original, driven by a
+new vanilla-JS module `payment-qr.js` that mirrors the `member-detail.js`
+convention from M6.5.
+
+All the building blocks are already in place:
+
+- **`/qr` endpoint** ships PNGs ([go/internal/web/qr.go](go/internal/web/qr.go), tested by [TestServeQR](go/internal/web/html_handler_test.go#L207-L231)).
+- **CSS** for `#qrModal .modal-content`, `.qr-image`, `.qr-details` is
+  already in [go/internal/web/static/css/app.css:445-478](go/internal/web/static/css/app.css#L445-L478) — lifted during M6.1.
+- **Template helpers** `qrHref` / `qrHrefAll` ([go/internal/web/render.go:63-85](go/internal/web/render.go#L63-L85))
+  already produce the correct query string + Czech `MM/YYYY` format
+  for the `message=` param. They're useful for sanity (and for any
+  no-JS fallback) but become dead code post-M6.6.1 — see §Out of scope.
+- **Bank account** is already exposed as `$.Data.BankAccount` to every
+  template that has a Pay button.
+- **JS module convention**: `member-detail.js` is the M6.5 baseline —
+  IIFE, `'use strict'`, event delegation off `data-*` attributes, no
+  inline `onclick`, no globals leaked.
+
+## Approach
+
+### 1. Replace `<a href="{{qrHref ...}}">` with `<button data-...>`
+
+In [go/internal/web/templates/adults.tmpl](go/internal/web/templates/adults.tmpl)
+and [go/internal/web/templates/juniors.tmpl](go/internal/web/templates/juniors.tmpl)
+(both have identical Pay markup at lines 65 and 72):
+
+```gotmpl
+<button type="button" class="pay-btn"
+        data-name="{{$row.Name}}"
+        data-amount="{{$cell.Amount}}"
+        data-month="{{$cell.Month}}"
+        data-raw-month="{{$cell.RawMonth}}">Pay</button>
+```
+
+```gotmpl
+<button type="button" class="pay-btn"
+        data-name="{{$row.Name}}"
+        data-amount="{{$row.PayableAmount}}"
+        data-month="{{$row.UnpaidPeriods}}"
+        data-raw-month="{{$row.RawUnpaidPeriods}}"
+        data-pay-all="1">Pay All</button>
+```
+
+Why `<button type="button">` and not `<a>`: the click no longer
+navigates — the JS handles it. `type="button"` prevents accidental form
+submission. A no-JS fallback could be added later by re-introducing
+`href="{{qrHref ...}}"` and letting the JS call `e.preventDefault()` —
+deferred until anyone actually asks (see §Out of scope).
+
+`data-month` carries the human-readable period (e.g. `January 2026`,
+or `January 2026 + February 2026` for Pay-All) which the modal title
+displays. `data-raw-month` carries the machine format (`2026-01`,
+`2026-01+2026-02`) which the JS converts to `MM/YYYY` for the QR
+message — same conversion Python's `showPayQR` does at
+[templates/adults.html:966-968](templates/adults.html#L966-L968).
+`data-pay-all` is purely informational; the JS doesn't actually need
+it (the same code path handles single + multi-period), but keeping it
+helps debugging and any future analytics.
+
+### 2. Add `#qrModal` markup
+
+Append after the `#memberModal` block in both `adults.tmpl` and
+`juniors.tmpl` (around line 185 of adults, line 165 of juniors), mirroring
+[templates/adults.html:631-648](templates/adults.html#L631-L648) but
+using `class="active"` toggling (the convention M6.5 picked) instead of
+`style.display = 'block'`:
+
+```gotmpl
+<div id="qrModal" onclick="if(event.target===this)this.classList.remove('active')">
+  <div class="modal-content">
+    <div class="modal-header">
+      <div class="modal-title" id="qrTitle">Payment for —</div>
+      <div class="close-btn" onclick="document.getElementById('qrModal').classList.remove('active')">[close]</div>
+    </div>
+    <div class="qr-image">
+      <img id="qrImg" src="" alt="Payment QR Code">
+    </div>
+    <div class="qr-details">
+      <div>Account: <span id="qrAccount"></span></div>
+      <div>Amount: <span id="qrAmount"></span> CZK</div>
+      <div>Message: <span id="qrMessage"></span></div>
+    </div>
+  </div>
+</div>
+```
+
+The CSS already targets `#qrModal .modal-content` directly. To make the
+"hidden by default / visible when `.active`" toggle behave the same
+way `#memberModal` does, the existing CSS rules for `#memberModal`
+(`display:none`, `#memberModal.active { display: flex; }`) need a
+sibling pair for `#qrModal`. Audit `app.css` during implementation:
+- if `#qrModal` already has both rules, no change.
+- if it has only `display:none`, add `#qrModal.active { display: flex; }`.
+- if it has neither, add both.
+
+This is a 2-line CSS adjustment at most; tracked under §Critical files.
+
+### 3. Bank account on the page
+
+The modal needs the bank account string for the `qrAccount` label and
+for building the `/qr?account=…` URL. Two clean options:
+
+**(a)** Read it from the `<button>`'s `data-bank-account` attribute.
+Per-button duplication, but trivially done in templates and keeps the JS
+module's surface tiny.
+
+**(b)** Stamp it once on the page as `<body data-bank-account="…">`
+(or on the existing `#filterContainer`).
+
+**Decision: (b)** — single stamping site, follows the same pattern
+M6.5 used for `data-page` on `#filterContainer`. Add
+`data-bank-account="{{.Data.BankAccount}}"` to `#filterContainer` in
+both templates (one-line edit each).
+
+### 4. Create [go/internal/web/static/js/payment-qr.js](go/internal/web/static/js/payment-qr.js)
+
+A new module loaded with `<script src="/static/js/payment-qr.js" defer></script>`
+right after `member-detail.js` in both templates. Pure ES, no build
+step, IIFE, `'use strict'`. Skeleton — port of Python's `showPayQR`
+([templates/adults.html:963-986](templates/adults.html#L963-L986))
+plus event wiring:
+
+```js
+(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 are converted piece-wise.
+  // Mirrors templates/adults.html:966-968.
+  function toCzechMonth(rawMonth) {
+    return rawMonth.split('+')
+      .map(p => p.replace(/^(\d{4})-(\d{2})$/, '$2/$1'))
+      .join('+');
+  }
+
+  function showQR(name, amount, month, rawMonth) {
+    const numericMonth = toCzechMonth(rawMonth);
+    const message = `${name}: ${numericMonth}`;
+
+    titleEl.innerText   = `Payment for ${month}`;
+    accountEl.innerText = bankAccount;
+    amountEl.innerText  = amount;
+    messageEl.innerText = message;
+
+    const url = '/qr?'
+      + 'account=' + encodeURIComponent(bankAccount)
+      + '&amount=' + encodeURIComponent(amount)
+      + '&message=' + encodeURIComponent(message);
+    imgEl.src = url;
+
+    modal.classList.add('active');
+  }
+
+  function closeQR() { modal.classList.remove('active'); }
+
+  // Event delegation: any .pay-btn click anywhere in the page.
+  document.addEventListener('click', ev => {
+    const 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', e => {
+    if (e.key === 'Escape' && modal.classList.contains('active')) closeQR();
+  });
+}());
+```
+
+Notes:
+
+- **Event delegation** (single listener on `document`) handles both Pay
+  and Pay-All without enumerating buttons. It also survives any future
+  re-render of the table without needing rebinding.
+- **Coexistence with `member-detail.js`**: `member-detail.js` listens
+  for `Escape` too, but only acts when `#memberModal` is active; the
+  new module similarly only acts when `#qrModal` is active. No collision.
+- **Pre-warming**: nothing to pre-warm — `/qr` is a tiny on-demand PNG
+  and only fires when the user clicks.
+- **Inline `onclick`** on `[close]` and outside-click in the markup are
+  carried over from the M6.5 modal style; consistent with `#memberModal`.
+  (Switching the project's modal close pattern to data attributes is a
+  separate cleanup, out of scope here.)
+
+### 5. Tests
+
+Extend [go/internal/web/html_handler_test.go](go/internal/web/html_handler_test.go)
+with markup-level checks. Headless-browser JS is out of scope (same
+rationale as M6.5).
+
+In `TestModalMarkup` (or a sibling test, e.g. `TestPaymentQRMarkup`):
+
+- Body for `/adults` and `/juniors` contains
+  `<script src="/static/js/payment-qr.js"`.
+- Body contains `id="qrModal"`, `id="qrImg"`, `id="qrAccount"`,
+  `id="qrAmount"`, `id="qrMessage"`, `id="qrTitle"`.
+- The fixture row's Pay button is rendered as
+  `<button … class="pay-btn" … data-name="Test Member" data-amount="…" data-raw-month="2026-01"`.
+- The fixture row's `.pay-btn` does **not** include `href=`.
+- `#filterContainer` has `data-bank-account="CZ0000000000000000000000"`
+  (matching the test's `fixtureHandler` config).
+
+### 6. Decide fate of `qrHref` / `qrHrefAll` template helpers
+
+Once Pay/Pay-All buttons are JS-driven, the helpers in
+[render.go:63-85](go/internal/web/render.go#L63-L85) have no callers.
+**Decision: delete them**, plus their entries in `tmplFuncs`. A
+no-JS fallback is not on the roadmap; if it ever becomes one, the
+helpers are easy to resurrect from git.
+
+The unit-test file currently has no direct tests for these helpers —
+nothing to remove from tests.
+
+## Critical files
+
+| Action | Path |
+|--------|------|
+| edit   | [go/internal/web/templates/adults.tmpl](go/internal/web/templates/adults.tmpl) — Pay/Pay-All → `<button data-…>`; add `#qrModal`; add `data-bank-account` on filter container; add `<script src="/static/js/payment-qr.js" defer>` |
+| edit   | [go/internal/web/templates/juniors.tmpl](go/internal/web/templates/juniors.tmpl) — same |
+| new    | `go/internal/web/static/js/payment-qr.js` — modal module |
+| edit   | [go/internal/web/render.go](go/internal/web/render.go) — drop `qrHref`, `qrHrefAll`, and their `tmplFuncs` entries (and the `net/url`, `strconv` imports if no longer needed) |
+| edit   | [go/internal/web/static/css/app.css](go/internal/web/static/css/app.css) — verify / add `#qrModal { display:none; }` + `#qrModal.active { display:flex; }` rules |
+| edit   | [go/internal/web/html_handler_test.go](go/internal/web/html_handler_test.go) — markup assertions |
+
+No changes to: `assets.go` (`embed.FS` glob picks up new JS automatically),
+`server.go`, `html_handler.go`, the `/qr` handler, `cmd/fuj/main.go`, the
+API handlers, or any domain code.
+
+## Reused existing infrastructure
+
+- [go/internal/web/qr.go](go/internal/web/qr.go) — `/qr?account=…&amount=…&message=…` endpoint serves the PNG already
+- [go/internal/web/static/css/app.css:445-478](go/internal/web/static/css/app.css#L445-L478) — modal CSS already lifted whole
+- [go/internal/web/static/js/member-detail.js](go/internal/web/static/js/member-detail.js) — IIFE / `'use strict'` / `data-*` style to mirror
+- [go/internal/web/api/types.go](go/internal/web/api/types.go) — `MemberRow.UnpaidPeriods` and `RawUnpaidPeriods` already populated for Pay-All
+- [templates/adults.html:631-648,963-986](templates/adults.html#L631-L648) — Python reference for both modal markup and `showPayQR` JS (logic to port, not literal copy)
+
+## Verification
+
+End-to-end, after copying the plan into `docs/plans/`:
+
+1. `cd go && make go-build go-test go-lint` — all green.
+2. `make web-go &` — boot Go on `:8080`.
+3. Browser: `http://localhost:8080/adults`
+   - Click `Pay` on any unpaid month → `#qrModal` opens with
+     - title `Payment for <Month YYYY>`,
+     - `qrAccount` matching `BANK_ACCOUNT`,
+     - `qrAmount` matching the cell amount,
+     - `qrMessage` formatted as `<Name>: MM/YYYY`,
+     - QR image rendered from `/qr?…` (Network tab shows PNG response).
+   - Tab does **not** navigate; URL stays `/adults`.
+   - Click `Pay All` → modal opens with `+`-joined `MM/YYYY+MM/YYYY` message.
+   - Click outside the modal-content → modal closes.
+   - Click `[close]` → modal closes.
+   - Press `Esc` → modal closes.
+   - Open `[i]` member-detail modal first, then `Esc` → only that closes
+     (verify both modals don't fight each other).
+4. Browser: `http://localhost:8080/juniors` — same checks. Verify Pay
+   on a `"?"`-expected row doesn't appear (Pay button is conditional on
+   `unpaid`/`partial` cells).
+5. Cross-check against Python: `make web-py &` →
+   `http://localhost:5001/adults` modal looks visually equivalent for
+   the same fixture (modulo any nav / theme differences already accepted
+   in M6).
+6. Scan a QR with a Czech banking app (or `zbarimg` on the saved PNG) —
+   payload starts with `SPD*1.0*ACC:` and the message decodes to
+   `<Name>: MM/YYYY`.
+7. After the user merges in Gitea: append CHANGELOG entry (timestamp via
+   `date "+%Y-%m-%d %H:%M %Z"`); add a new tick line for **M6.6.1** in
+   [docs/plans/2026-05-03-2349-go-backend-rewrite-progress.md](docs/plans/2026-05-03-2349-go-backend-rewrite-progress.md)
+   immediately under the M6.6 line, in the same format as the other
+   sub-bullets.
+
+## Branching
+
+Per [CLAUDE.md](CLAUDE.md): create `feat/go-m6-6-1-payment-qr-modal` off
+`main`, commit with `Co-Authored-By` trailer, push with `-u`, open MR via
+`tea pr create`. Do not merge or delete from CLI.
+
+## Out of scope (deferred)
+
+- **Pay button no-JS fallback** — feasible to keep `qrHref` and have
+  the JS `preventDefault` an `<a>`, but no current need. Resurrect the
+  helpers if asked.
+- **Modal pattern unification** — `#memberModal` and `#qrModal` use
+  inline `onclick` for `[close]` and outside-click. Consistent with
+  M6.5 but worth a follow-up to delegate-event everything.
+- **Pay-from-modal inside `#memberModal`** — Python doesn't have it, and
+  no one's asked for it on the Go side.
+- **Per-button QR error-correction / size** — `/qr` is fixed at
+  Medium/256, same as M6.6. Configurable via query param can be added if
+  printing larger physical posters becomes a use case.
+- **`/payments` Pay buttons** — the payments page is a ledger view; it
+  doesn't render Pay buttons in either backend.
diff --git a/go/internal/web/html_handler_test.go b/go/internal/web/html_handler_test.go
index 8366afe..23b2b5b 100644
--- a/go/internal/web/html_handler_test.go
+++ b/go/internal/web/html_handler_test.go
@@ -314,6 +314,52 @@ func TestServeSync(t *testing.T) {
 	}
 }
 
+func TestPaymentQRMarkup(t *testing.T) {
+	renderer, err := web.NewRenderer()
+	if err != nil {
+		t.Fatalf("NewRenderer: %v", err)
+	}
+	b := web.BuildInfo{Version: "v0", Commit: "abc1234", BuildDate: "2026-01-01"}
+	h := web.NewHTMLHandler(renderer, b, fixtureHandler(t), web.ActionHandlers{})
+
+	cases := []struct {
+		path    string
+		handler http.HandlerFunc
+	}{
+		{"/adults", h.ServeAdults},
+		{"/juniors", h.ServeJuniors},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.path, func(t *testing.T) {
+			req := httptest.NewRequest(http.MethodGet, tc.path, nil)
+			w := httptest.NewRecorder()
+			tc.handler(w, req)
+			body := w.Body.String()
+
+			for _, want := range []string{
+				`id="qrModal"`,
+				`id="qrImg"`,
+				`id="qrTitle"`,
+				`id="qrAccount"`,
+				`id="qrAmount"`,
+				`id="qrMessage"`,
+				`/static/js/payment-qr.js`,
+				`data-bank-account="CZ0000000000000000000000"`,
+			} {
+				if !strings.Contains(body, want) {
+					t.Errorf("body missing %q", want)
+				}
+			}
+
+			// Pay buttons must use <button>, never a bare href to /qr.
+			if strings.Contains(body, `href="/qr`) {
+				t.Error("body must not contain href=/qr (Pay buttons should be <button>, not <a>)")
+			}
+		})
+	}
+}
+
 func TestServeVersion(t *testing.T) {
 	req := httptest.NewRequest(http.MethodGet, "/api/version", nil)
 	w := httptest.NewRecorder()
diff --git a/go/internal/web/render.go b/go/internal/web/render.go
index b30c3c9..6990a1c 100644
--- a/go/internal/web/render.go
+++ b/go/internal/web/render.go
@@ -6,8 +6,6 @@ import (
 	"html/template"
 	"log/slog"
 	"net/http"
-	"net/url"
-	"strconv"
 )
 
 // PageData is the view model passed to every HTML template.
@@ -58,36 +56,7 @@ type Renderer struct {
 
 var pageNames = []string{"adults", "juniors", "payments", "sync", "flush_cache"}
 
-// qrHref builds the /qr query URL for a single-month Pay button.
-// rawMonth is "YYYY-MM"; it is converted to "MM/YYYY" in the QR message.
-func qrHref(account string, amount int, name, rawMonth string) string {
-	// Convert "YYYY-MM" → "MM/YYYY" to match Python's showPayQR JS.
-	if len(rawMonth) == 7 && rawMonth[4] == '-' {
-		rawMonth = rawMonth[5:] + "/" + rawMonth[:4]
-	}
-	msg := name + ": " + rawMonth
-	return "/qr?" + url.Values{
-		"account": {account},
-		"amount":  {strconv.Itoa(amount)},
-		"message": {msg},
-	}.Encode()
-}
-
-// qrHrefAll builds the /qr query URL for a Pay-All button.
-// rawPeriods is the "+" -joined MM/YYYY string from MemberRow.RawUnpaidPeriods.
-func qrHrefAll(account string, amount int, name, rawPeriods string) string {
-	msg := name + ": " + rawPeriods
-	return "/qr?" + url.Values{
-		"account": {account},
-		"amount":  {strconv.Itoa(amount)},
-		"message": {msg},
-	}.Encode()
-}
-
-var tmplFuncs = template.FuncMap{
-	"qrHref":    qrHref,
-	"qrHrefAll": qrHrefAll,
-}
+var tmplFuncs = template.FuncMap{}
 
 // NewRenderer parses all templates from the embedded FS.
 // A parse failure should be treated as a startup-time fatal error.
diff --git a/go/internal/web/static/css/app.css b/go/internal/web/static/css/app.css
index ff30e45..8de9b2d 100644
--- a/go/internal/web/static/css/app.css
+++ b/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;
diff --git a/go/internal/web/static/js/payment-qr.js b/go/internal/web/static/js/payment-qr.js
new file mode 100644
index 0000000..b9af686
--- /dev/null
+++ b/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();
+  });
+}());
diff --git a/go/internal/web/templates/adults.tmpl b/go/internal/web/templates/adults.tmpl
index bed744e..2a9129c 100644
--- a/go/internal/web/templates/adults.tmpl
+++ b/go/internal/web/templates/adults.tmpl
@@ -12,7 +12,7 @@
   <a href="{{.Data.PaymentsURL}}" target="_blank" rel="noopener">Payments Ledger</a>
 </div>
 
-<div class="filter-container" id="filterContainer" data-current-month="{{.Data.CurrentMonth}}" data-page="adults">
+<div class="filter-container" id="filterContainer" data-current-month="{{.Data.CurrentMonth}}" data-page="adults" data-bank-account="{{.Data.BankAccount}}">
   <div class="filter-item">
     <label class="filter-label" for="nameFilter">Member</label>
     <input id="nameFilter" class="filter-input" type="text" placeholder="Filter by name…">
@@ -62,14 +62,14 @@
           class="{{if eq $cell.Status "empty"}}cell-empty{{else if and (or (eq $cell.Status "unpaid") (eq $cell.Status "partial")) (ge $cell.RawMonth $.Data.CurrentMonth)}}cell-unpaid-current{{else if or (eq $cell.Status "unpaid") (eq $cell.Status "partial")}}cell-unpaid{{else if eq $cell.Status "ok"}}cell-ok{{end}}{{if $cell.Overridden}} cell-overridden{{end}}">
         {{$cell.Text}}
         {{if and (or (eq $cell.Status "unpaid") (eq $cell.Status "partial")) (lt $cell.RawMonth $.Data.CurrentMonth)}}
-        <a class="pay-btn" href="{{qrHref $.Data.BankAccount $cell.Amount $row.Name $cell.RawMonth}}">Pay</a>
+        <button type="button" class="pay-btn" data-name="{{$row.Name}}" data-amount="{{$cell.Amount}}" data-month="{{$cell.Month}}" data-raw-month="{{$cell.RawMonth}}">Pay</button>
         {{end}}
       </td>
       {{end}}
       <td class="{{if lt $row.Balance 0}}balance-neg{{else if gt $row.Balance 0}}balance-pos{{end}}" style="position: relative;">
         {{$row.Balance}}
         {{if gt $row.PayableAmount 0}}
-        <a class="pay-btn" href="{{qrHrefAll $.Data.BankAccount $row.PayableAmount $row.Name $row.RawUnpaidPeriods}}">Pay All</a>
+        <button type="button" class="pay-btn" data-name="{{$row.Name}}" data-amount="{{$row.PayableAmount}}" data-month="{{$row.UnpaidPeriods}}" data-raw-month="{{$row.RawUnpaidPeriods}}" data-pay-all="1">Pay All</button>
         {{end}}
       </td>
     </tr>
@@ -184,6 +184,24 @@
   </div>
 </div>
 
+<div id="qrModal" onclick="if(event.target===this)this.classList.remove('active')">
+  <div class="modal-content">
+    <div class="modal-header">
+      <div class="modal-title" id="qrTitle">Payment for —</div>
+      <div class="close-btn" onclick="document.getElementById('qrModal').classList.remove('active')">[close]</div>
+    </div>
+    <div class="qr-image">
+      <img id="qrImg" src="" alt="Payment QR Code">
+    </div>
+    <div class="qr-details">
+      <div>Account: <span id="qrAccount"></span></div>
+      <div>Amount: <span id="qrAmount"></span> CZK</div>
+      <div>Message: <span id="qrMessage"></span></div>
+    </div>
+  </div>
+</div>
+
 <script src="/static/js/filters.js" defer></script>
 <script src="/static/js/member-detail.js" defer></script>
+<script src="/static/js/payment-qr.js" defer></script>
 {{end}}
diff --git a/go/internal/web/templates/juniors.tmpl b/go/internal/web/templates/juniors.tmpl
index 5981ffc..0f794b4 100644
--- a/go/internal/web/templates/juniors.tmpl
+++ b/go/internal/web/templates/juniors.tmpl
@@ -12,7 +12,7 @@
   <a href="{{.Data.PaymentsURL}}" target="_blank" rel="noopener">Payments Ledger</a>
 </div>
 
-<div class="filter-container" id="filterContainer" data-current-month="{{.Data.CurrentMonth}}" data-page="juniors">
+<div class="filter-container" id="filterContainer" data-current-month="{{.Data.CurrentMonth}}" data-page="juniors" data-bank-account="{{.Data.BankAccount}}">
   <div class="filter-item">
     <label class="filter-label" for="nameFilter">Member</label>
     <input id="nameFilter" class="filter-input" type="text" placeholder="Filter by name…">
@@ -62,14 +62,14 @@
           class="{{if eq $cell.Status "empty"}}cell-empty{{else if and (or (eq $cell.Status "unpaid") (eq $cell.Status "partial")) (ge $cell.RawMonth $.Data.CurrentMonth)}}cell-unpaid-current{{else if or (eq $cell.Status "unpaid") (eq $cell.Status "partial")}}cell-unpaid{{else if eq $cell.Status "ok"}}cell-ok{{end}}{{if $cell.Overridden}} cell-overridden{{end}}">
         {{$cell.Text}}
         {{if and (or (eq $cell.Status "unpaid") (eq $cell.Status "partial")) (lt $cell.RawMonth $.Data.CurrentMonth)}}
-        <a class="pay-btn" href="{{qrHref $.Data.BankAccount $cell.Amount $row.Name $cell.RawMonth}}">Pay</a>
+        <button type="button" class="pay-btn" data-name="{{$row.Name}}" data-amount="{{$cell.Amount}}" data-month="{{$cell.Month}}" data-raw-month="{{$cell.RawMonth}}">Pay</button>
         {{end}}
       </td>
       {{end}}
       <td class="{{if lt $row.Balance 0}}balance-neg{{else if gt $row.Balance 0}}balance-pos{{end}}" style="position: relative;">
         {{$row.Balance}}
         {{if gt $row.PayableAmount 0}}
-        <a class="pay-btn" href="{{qrHrefAll $.Data.BankAccount $row.PayableAmount $row.Name $row.RawUnpaidPeriods}}">Pay All</a>
+        <button type="button" class="pay-btn" data-name="{{$row.Name}}" data-amount="{{$row.PayableAmount}}" data-month="{{$row.UnpaidPeriods}}" data-raw-month="{{$row.RawUnpaidPeriods}}" data-pay-all="1">Pay All</button>
         {{end}}
       </td>
     </tr>
@@ -164,6 +164,24 @@
   </div>
 </div>
 
+<div id="qrModal" onclick="if(event.target===this)this.classList.remove('active')">
+  <div class="modal-content">
+    <div class="modal-header">
+      <div class="modal-title" id="qrTitle">Payment for —</div>
+      <div class="close-btn" onclick="document.getElementById('qrModal').classList.remove('active')">[close]</div>
+    </div>
+    <div class="qr-image">
+      <img id="qrImg" src="" alt="Payment QR Code">
+    </div>
+    <div class="qr-details">
+      <div>Account: <span id="qrAccount"></span></div>
+      <div>Amount: <span id="qrAmount"></span> CZK</div>
+      <div>Message: <span id="qrMessage"></span></div>
+    </div>
+  </div>
+</div>
+
 <script src="/static/js/filters.js" defer></script>
 <script src="/static/js/member-detail.js" defer></script>
+<script src="/static/js/payment-qr.js" defer></script>
 {{end}}