refactor: update theme management and enhance UI components

- Refactored theme imports in `preferences.ts` and `ThemeSelector.tsx` to use centralized `theme-config`.
- Added new CSS variables for special cards in `globals.css` to improve theme consistency.
- Enhanced `Header` and `TaskCard` components with theme dropdown functionality for better user experience.
- Updated `ThemeProvider` to support cycling through dark themes, improving theme selection flexibility.
- Cleaned up unused imports and streamlined component structures for better maintainability.

src/components/ui/index.ts

@@ -24,5 +24,4 @@ export { DropZone } from './DropZone';
 
 // Composants existants
 export { Card, CardHeader, CardTitle, CardContent, CardFooter } from './Card';
-export { Header } from './Header';
 export { FontSizeToggle } from './FontSizeToggle';