feat: enhance v2 components and entity join functionality

- Add entity join controller/routes enhancements
- Improve table management and category value services
- Update v2 table list, card display, search widget components
- Improve split panel layout responsiveness

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend-node/src/routes/entityJoinRoutes.ts

@@ -55,6 +55,15 @@ router.get(
   entityJoinController.getTableDataWithJoins.bind(entityJoinController)
 );
 
+/**
+ * 테이블 컬럼의 고유값 목록 조회 (헤더 필터 드롭다운용)
+ * GET /api/table-management/tables/:tableName/column-values/:columnName
+ */
+router.get(
+  "/tables/:tableName/column-values/:columnName",
+  entityJoinController.getColumnUniqueValues.bind(entityJoinController)
+);
+
 // ========================================
 // 🎯 Entity 조인 설정 관리
 // ========================================