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/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.