feat(go): M6.4 — Go-native /payments page (grouped-by-person ledger)

- Extract AssemblePayments(ctx) from ServePayments in api/handler.go,
  mirroring the AssembleAdults/AssembleJuniors pattern
- Add PaymentsPageData view-model wrapper in render.go
- Rewire html_handler.go ServePayments to call AssemblePayments and
  render with PaymentsPageData
- Replace payments.tmpl placeholder with real grouped-by-person ledger:
  alphabetical member blocks, txn-table (Date/Amount/Purpose/Message),
  newest-first rows, Unmatched/Unknown bucket
- Append ledger CSS classes to app.css (.ledger-container, .member-block,
  .txn-table, .txn-date/amount/purpose/message, tr:hover)
- Add TestPaymentsPage markup test

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

go/internal/web/html_handler.go

@@ -48,7 +48,18 @@ func (h *HTMLHandler) ServeJuniors(w http.ResponseWriter, r *http.Request) {
 }
 
 func (h *HTMLHandler) ServePayments(w http.ResponseWriter, r *http.Request) {
-	h.renderer.Render(w, "payments", PageData{Active: "payments", Build: h.build})
+	data, err := h.apiHandler.AssemblePayments(r.Context())
+	if err != nil {
+		h.renderer.Render(w, "payments", PaymentsPageData{
+			PageData: PageData{Active: "payments", Build: h.build},
+			Error:    err.Error(),
+		})
+		return
+	}
+	h.renderer.Render(w, "payments", PaymentsPageData{
+		PageData: PageData{Active: "payments", Build: h.build},
+		Data:     data,
+	})
 }
 
 func (h *HTMLHandler) ServeSync(w http.ResponseWriter, r *http.Request) {