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/render.go

@@ -30,6 +30,13 @@ type JuniorsPageData struct {
 	Error string
 }
 
+// PaymentsPageData is the view model for the /payments HTML page.
+type PaymentsPageData struct {
+	PageData
+	Data  api.PaymentsResponse
+	Error string
+}
+
 // Renderer parses and executes HTML templates from the embedded FS.
 type Renderer struct {
 	tmpls map[string]*template.Template