Goal for this part
Add toolbar actions to create, edit, and delete books through modal dialogs that write to SQLite.
Concepts: Toplevel dialogs
A Toplevel window with grab_set() acts like a modal dialog. Keep validation in the dialog; keep SQL in the main app methods. Refuse to delete a title that still has open loans.
How to run it and what you should see
Add a book, edit its title, then try deleting a title with open loans and confirm the app blocks it.