feat(gantt): introduce advanced Gantt chart components and demo page

- Added new components for an advanced Gantt chart, including AdvancedGanttChart, GanttGrid, DraggableTaskBar, and TimelineMarkers.
- Implemented a demo page (GanttDemoPage) to showcase the functionality of the new Gantt chart components.
- Enhanced project roadmap features with ProjectRoadmapGantt and related components for better project management visualization.
- Introduced sample data for testing and demonstration purposes, improving the user experience in the Gantt chart interface.
- Updated main routes to include the new Gantt demo page for easy access.

worklenz-frontend/src/pages/GanttDemoPage.tsx

@@ -0,0 +1,8 @@
+import React from 'react';
+import { AdvancedGanttDemo } from '../components/advanced-gantt';
+
+const GanttDemoPage: React.FC = () => {
+  return <AdvancedGanttDemo />;
+};
+
+export default GanttDemoPage;