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/templates/payments.tmpl

@@ -1,5 +1,38 @@
 {{define "title"}}Payments Ledger{{end}}
 {{define "content"}}
 <h1>Payments Ledger</h1>
-<p class="description">Coming in M6.4</p>
+<p class="description">
+    All bank transactions from the
+    <a href="{{.Data.PaymentsURL}}" target="_blank">payments sheet</a>,
+    grouped by member. Names are matched against the
+    <a href="{{.Data.AttendanceURL}}" target="_blank">attendance sheet</a>.
+</p>
+{{if .Error}}<p class="error">{{.Error}}</p>{{end}}
+<div class="ledger-container">
+{{range $person := .Data.SortedPeople}}
+    <div class="member-block">
+        <h2>{{$person}}</h2>
+        <table class="txn-table">
+            <thead>
+                <tr>
+                    <th class="txn-date">Date</th>
+                    <th class="txn-amount">Amount</th>
+                    <th class="txn-purpose">Purpose</th>
+                    <th class="txn-message">Bank Message</th>
+                </tr>
+            </thead>
+            <tbody>
+            {{range $tx := index $.Data.GroupedPayments $person}}
+                <tr>
+                    <td class="txn-date">{{$tx.Date}}</td>
+                    <td class="txn-amount">{{printf "%.0f" $tx.Amount}} CZK</td>
+                    <td class="txn-purpose">{{$tx.Purpose}}</td>
+                    <td class="txn-message">{{$tx.Message}}</td>
+                </tr>
+            {{end}}
+            </tbody>
+        </table>
+    </div>
+{{end}}
+</div>
 {{end}}