feat: snackbar on book instance save

lib/ui/add_page/widgets/confirmation_popup.dart

@@ -143,6 +143,12 @@ class _ConfirmationPopupState extends State<ConfirmationPopup> {
                       TextButton(
                         onPressed: () {
                           widget.exitPopup(context);
+                          ScaffoldMessenger.of(context).showSnackBar(
+                            SnackBar(
+                              content: Text("Enregistré: ${widget.book.title}"),
+                              behavior: SnackBarBehavior.floating,
+                            ),
+                          );
                         },
                         child: Text("Ok"),
                       ),