feat: enhance JiraDashboardPage with new components and improved UI

- Integrated `PeriodSelector`, `SkeletonGrid`, and `MetricsGrid` for better data visualization and user interaction.
- Replaced legacy period selection and error display with new components for a cleaner UI.
- Updated `UIShowcaseClient` to demonstrate new Jira dashboard components, enhancing showcase functionality.

src/components/ui/index.ts

@@ -35,6 +35,11 @@ export { DailyAddForm } from './DailyAddForm';
 export { AlertBanner } from './AlertBanner';
 export { CollapsibleSection } from './CollapsibleSection';
 
+// Composants Jira Dashboard
+export { PeriodSelector } from './PeriodSelector';
+export { SkeletonCard, SkeletonGrid } from './SkeletonCard';
+export { MetricsGrid } from './MetricsGrid';
+
 // Composants existants
 export { Card, CardHeader, CardTitle, CardContent, CardFooter } from './Card';
 export { FontSizeToggle } from './FontSizeToggle';