修复振幅抽屉计算方向，DESC 排序时取后5行（更老数据）

ft-app/app/templates/contract.html

@@ -87,14 +87,15 @@ document.querySelectorAll('.amp-cell').forEach(function(cell) {
     // Only rows with data-index (skip header + next-day row)
     var dataRows = tbl.querySelectorAll('tr[data-index]');
     var idx = parseInt(row.dataset.index);
-    if (idx < 5) return;
+    // Need 5 older rows (higher index, since newest first)
+    if (idx + 5 >= dataRows.length) return;
 
     document.getElementById('drawer-title').textContent = '{{ contract }}';
     document.getElementById('drawer-date').textContent = '目标: ' + row.dataset.date + ' ' + row.dataset.weekday + '  振幅 ' + cell.textContent.trim();
 
     var html = '<tr><td>日期</td><td>最高−最低</td></tr>';
     var sum = 0;
-    for (var j = idx - 5; j < idx; j++) {
+    for (var j = idx + 1; j <= idx + 5; j++) {
       var r = dataRows[j];
       html += '<tr><td>' + r.dataset.date + ' ' + r.dataset.weekday + '</td><td>' + r.dataset.diff + '</td></tr>';
       sum += parseInt(r.dataset.diff);