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/static/css/app.css

@@ -476,3 +476,59 @@ tr:hover {
     color: #00ff00;
     font-family: monospace;
 }
+
+/* /payments ledger */
+.ledger-container {
+    width: 100%;
+    max-width: 1000px;
+    margin-bottom: 40px;
+}
+
+.member-block {
+    margin-bottom: 20px;
+}
+
+.txn-table {
+    border-collapse: collapse;
+    width: 100%;
+    margin-top: 5px;
+}
+
+.txn-table th,
+.txn-table td {
+    padding: 2px 8px;
+    text-align: left;
+    border-bottom: 1px dashed #222;
+}
+
+.txn-table th {
+    color: #555;
+    text-transform: lowercase;
+    font-weight: normal;
+    border-bottom: 1px solid #333;
+}
+
+.txn-date {
+    min-width: 80px;
+    color: #888;
+}
+
+.txn-amount {
+    min-width: 80px;
+    text-align: right !important;
+    color: #00ff00;
+}
+
+.txn-purpose {
+    min-width: 100px;
+    color: #aaa;
+}
+
+.txn-message {
+    color: #666;
+    font-style: italic;
+}
+
+.txn-table tr:hover {
+    background-color: #1a1a1a;
+}