Enhance the ergonomy of contextual menu in Gantt

Currently, contextual menu are displayed when clicking a task a Gantt diagram.

<img width="396" height="141" alt="Image" src="https://github.com/user-attachments/assets/4e970389-8d35-4331-84ba-02852efd9cbc" />

## Expected

We should have the contextual menu displayed when right-clicking on the task.
And also right-clicking on the background of the Gantt diagram : refer to #20 
