style: enhance video presentation with ambient backdrop and rounded shapes
- Introduce a new class for video media visualization in embed.css, applying rounded shapes and gradient layers for a cohesive look.
- Update video and thumbnail styles to improve layering and z-index management for better visual hierarchy.
- Implement a video ambient backdrop in shared.css to match the app's aesthetic, enhancing the overall user experience.
- Refactor related styles in post.css and profile.css to maintain consistency with the new design approach.