feat(drug-usage): 음수재고 빨간색 + 테이블 스크롤 고정헤더

- 음수 현재고 빨간색 강조 (.negative-stock)
- 입고/조제 테이블 sticky header (max-height: 300px)
- 테이블 wrapper 추가 (border-radius)

backend/templates/admin_drug_usage.html

@@ -263,6 +263,10 @@
         .num-secondary {
             color: #3b82f6;
         }
+        .negative-stock {
+            color: #dc2626 !important;
+            font-weight: 600;
+        }
 
         /* ── 빈 상태 ── */
         .empty-state {
@@ -390,6 +394,12 @@
             background: #fef3c7;
             color: #92400e;
         }
+        .detail-table-wrapper {
+            max-height: 300px;
+            overflow-y: auto;
+            border: 1px solid #e2e8f0;
+            border-radius: 6px;
+        }
         .detail-table {
             width: 100%;
             border-collapse: separate;
@@ -398,6 +408,13 @@
             min-width: 0;
             table-layout: fixed;
         }
+        .detail-table thead th {
+            position: sticky;
+            top: 0;
+            background: #f8fafc;
+            z-index: 1;
+            box-shadow: 0 1px 0 #e2e8f0;
+        }
         .detail-table td, .detail-table th {
             padding: 4px 8px;
             border-bottom: 1px solid #e0e0e0;
@@ -745,7 +762,7 @@
                 <td class="num-cell num-secondary">${formatNumber(item.import_count)}</td>
                 <td class="num-cell num-highlight">${formatNumber(item.rx_total_qty)}</td>
                 <td class="num-cell num-secondary">${formatNumber(item.import_total_qty)}</td>
-                <td class="num-cell">${formatNumber(item.current_stock)}</td>
+                <td class="num-cell ${item.current_stock < 0 ? 'negative-stock' : ''}">${formatNumber(item.current_stock)}</td>
             </tr>
             <tr class="detail-row" id="detail-${escapeHtml(item.drug_code)}" style="display:${expandedDrugCode === item.drug_code ? 'table-row' : 'none'};">
                 <td colspan="8">
@@ -811,6 +828,7 @@
             if (data.success && data.items.length > 0) {
                 container.innerHTML = `
                     <h4>📦 입고목록 <span class="count">(${data.total_count}건)</span></h4>
+                    <div class="detail-table-wrapper">
                     <table class="detail-table" style="table-layout:fixed;">
                         <colgroup>
                             <col style="width:70px;">
@@ -843,6 +861,7 @@
                             `).join('')}
                         </tbody>
                     </table>
+                    </div>
                 `;
             } else {
                 container.innerHTML = `
@@ -886,6 +905,7 @@
                 
                 container.innerHTML = `
                     <h4>💊 조제목록 <span class="count">(${data.total_count}건)</span> <span class="patient-info">${patientBadges}</span></h4>
+                    <div class="detail-table-wrapper">
                     <table class="detail-table" style="table-layout:fixed;">
                         <colgroup>
                             <col style="width:65px;">
@@ -924,6 +944,7 @@
                             `).join('')}
                         </tbody>
                     </table>
+                    </div>
                 `;
             } else {
                 container.innerHTML = `