Fix display tab "New page" on fresh project

[kubiix]

This pull request refactors the logic for ensuring a project has at least one page by moving it from the UI layer to the model layer. The change improves code organization and encapsulation by centralizing this responsibility within the Project class.

Refactoring and Encapsulation:

• Added the AssureFirstPage method to the Project class (Yafc.Model/Model/Project.cs) to handle the creation and initialization of the first page and its display mapping if none exist.
• Updated the SetProject method in MainScreen.cs to call project.AssureFirstPage() instead of duplicating page initialization logic, removing redundant code from the UI layer.

[veger]

Thank you for your contribution!
The code change looks fine to me.

Please add a changelog entry if you want, otherwise we can do it for you as well.

[kubiix]

Changelog filled

[DaleStan]

Thanks! That's been bothering me for a while, but never enough to actually fix it.

Everything looks good except that 63b7078 appears to have been committed with core.autocrlf or core.eol not set correctly. Do you want to fix that, or would you rather I do it?

[shpaass]

Thank you for the fix! I'll merge this one after other reviewers are done.