feat: 맞춤 집계표 엑셀 다운로드 추가

- 조회 폼에 '엑셀저장' 버튼(현재 컬럼·필터·제목·집계방식 그대로 반영)
- Excel 2003 XML로 제목·기간·품목구분 머리행 + 선택 컬럼 + 합계행 출력

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/Views/admin/sales_report/custom_report.php

@@ -32,6 +32,7 @@ $nf = static fn ($n): string => number_format((int) $n);
 <section class="p-3 bg-white border-b border-gray-200 no-print">
   <form method="get" action="<?= mgmt_url('reports/custom') ?>" id="custom-report-form" class="space-y-3">
     <input type="hidden" name="applied" value="1"/>
+    <input type="hidden" name="export" id="cr-export" value="0"/>
     <div class="flex flex-wrap items-end gap-3 text-sm">
       <div>
         <label class="block text-xs font-bold text-gray-600 mb-1">표 제목</label>
@@ -60,7 +61,8 @@ $nf = static fn ($n): string => number_format((int) $n);
           <option value="summary" <?= $mode === 'summary' ? 'selected' : '' ?>>품목별 집계</option>
         </select>
       </div>
-      <button type="submit" class="bg-btn-search text-white px-4 py-1.5 rounded-sm text-sm shadow hover:opacity-90">조회</button>
+      <button type="submit" onclick="document.getElementById('cr-export').value='0'" class="bg-btn-search text-white px-4 py-1.5 rounded-sm text-sm shadow hover:opacity-90">조회</button>
+      <button type="submit" onclick="document.getElementById('cr-export').value='1'" class="border border-btn-excel-border text-btn-excel-text px-4 py-1.5 rounded-sm text-sm hover:bg-green-50 transition">엑셀저장</button>
     </div>
 
     <div>