feat: enhance Kanban components with new UI elements

- Added `ColumnHeader`, `EmptyState`, and `DropZone` components to improve the Kanban UI structure and user experience.
- Refactored `KanbanColumn` to utilize the new components, enhancing readability and maintainability.
- Updated `Card` component to support flexible props for shadow, border, and background, allowing for better customization across the application.
- Adjusted `SwimlanesBase` to incorporate the new `ColumnHeader` for consistent column representation.

src/components/ui/index.ts

@@ -18,6 +18,9 @@ export { SearchInput } from './SearchInput';
 export { ControlPanel, ControlSection, ControlGroup } from './ControlPanel';
 export { FilterSummary } from './FilterSummary';
 export { FilterChip } from './FilterChip';
+export { ColumnHeader } from './ColumnHeader';
+export { EmptyState } from './EmptyState';
+export { DropZone } from './DropZone';
 
 // Composants existants
 export { Card, CardHeader, CardTitle, CardContent, CardFooter } from './Card';