feat: improve theme selector and UI components

- Updated `ThemeSelector` to use a new `ThemePreview` component for better theme visualization.
- Refactored button implementation in `ThemeSelector` to utilize the new `Button` component, enhancing consistency.
- Added a UI showcase section in `GeneralSettingsPageClient` to display available UI components with different themes.
- Enhanced `Badge`, `Button`, and `Input` components with new variants and improved styling for better usability and visual appeal.
- Updated CSS variables in `globals.css` for improved contrast and accessibility across themes.

src/components/ui/index.ts

@@ -0,0 +1,11 @@
+// Composants UI de base
+export { Button } from './Button';
+export { Badge } from './Badge';
+export { Alert, AlertTitle, AlertDescription } from './Alert';
+export { Input } from './Input';
+export { StyledCard } from './StyledCard';
+
+// Composants existants
+export { Card, CardHeader, CardTitle, CardContent, CardFooter } from './Card';
+export { Header } from './Header';
+export { FontSizeToggle } from './FontSizeToggle';