fix(mobile): person birthday viewing/editing (#20731)

* fix: edit birthday dialog

* chore: convert age to "x years old" format

* fix: lint

---------

Co-authored-by: Alex <alex.tran1502@gmail.com>

mobile/lib/presentation/pages/drift_person.page.dart

@@ -64,6 +64,7 @@ class _DriftPersonPageState extends ConsumerState<DriftPersonPage> {
             await handleEditBirthday(context);
             context.pop();
           },
+          birthdayExists: _person.birthDate != null,
         );
       },
     );