feat: Update SplitPanelLayoutComponent to manage custom left selected data

- Initialized custom left selected data to an empty object when deselecting an item, ensuring a clean state for the right form.
- Passed the selected item to the custom left selected data when an item is selected, improving data handling in custom mode.
- Enhanced overall data management within the SplitPanelLayoutComponent for better user experience.

frontend/lib/registry/components/v2-split-panel-layout/SplitPanelLayoutComponent.tsx

@@ -1564,6 +1564,7 @@ export const SplitPanelLayoutComponent: React.FC<SplitPanelLayoutComponentProps>
       if (isSameItem) {
         // 선택 해제 → 전체 데이터 로드
         setSelectedLeftItem(null);
+        setCustomLeftSelectedData({}); // 커스텀 모드 우측 폼 데이터 초기화
         setExpandedRightItems(new Set());
         setTabsData({});
         if (activeTabIndex === 0) {
@@ -1584,6 +1585,7 @@ export const SplitPanelLayoutComponent: React.FC<SplitPanelLayoutComponentProps>
       }
 
       setSelectedLeftItem(item);
+      setCustomLeftSelectedData(item); // 커스텀 모드 우측 폼에 선택된 데이터 전달
       setExpandedRightItems(new Set()); // 좌측 항목 변경 시 우측 확장 초기화
       setTabsData({}); // 모든 탭 데이터 초기화