Changes

Switch from `XapiGroup` to `Clazz` for student group selection and update related logic.

- Refactor `AssignmentEditScreen` and `AssignmentEditViewModel` to use `Clazz` options instead of `XapiGroup` for the assignee field.
- Update `UpdateClazzStudentXapiGroupUseCase` to resolve class names via xAPI activity statements rather than direct class lookup.
- Add debug logging to `ClazzDetailViewModel` and `UpdateClazzStudentXapiGroupUseCase` to track group member synchronization.
- Map `Clazz` objects to xAPI groups using `studentsXapiGroup` extension when selecting an assignee.

Refactor `AssignmentEditViewModel` to use `XapiGroup` for class selection and remove debug logs in `ClazzDetailViewModel`.

testing.