Next, Previous Button Alignment Improvement

[mariajgrimaldi]

Description

On course content pages, the "Next" and "Previous" navigation buttons are not center-aligned at the bottom as expected. This misalignment affects the user experience and consistency across the course flow.

Steps to Reproduce

1. Log in to the platform.
2. Open any course (e.g., Demo course).
3. Scroll to the bottom of a lesson or content page.
4. Observe the positioning of the "Next" and "Previous" buttons.

openedx/wg-build-test-release#460