feat(admin): AdminSidebar에 "팩 자료" 메뉴 추가

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/admin/components/AdminSidebar.tsx

@@ -68,6 +68,16 @@ const NAV_ITEMS = [
       </svg>
     ),
   },
+  {
+    href: '/admin/packs',
+    label: '팩 자료',
+    icon: (
+      <svg className="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+        <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2}
+          d="M20 7l-8-4-8 4m16 0l-8 4m8-4v10l-8 4m0-10L4 7m8 4v10M4 7v10l8 4" />
+      </svg>
+    ),
+  },
   {
     href: '/admin/questionnaire',
     label: '질문지 응답',