Display warning message when closing a project

[AmelieAnnemie]

• [ ] The feature was discussed in discussions of this repository.
• [x] I have searched the issues of this repository and believe that this is not a duplicate.

Description 💡

Add a warning message when the user closes a project:

Example wording:

If you close the project, you will no longer be able to make changes or add information.

This warning message should be displayed for projects and sub-projects.

Please adapt in all language versions.