Use object-fit for demos

[object-fit](https://developer.mozilla.org/en-US/docs/Web/CSS/object-fit)

The object-fit CSS property specifies how the contents of a replaced element, such as an or , should be resized to fit its container.

Start with the linked MDN reference for the object-fit property, then inspect the repository's demo markup and styles to find the affected images or videos. The issue provides no file or test location; the work is done when the relevant demos use object-fit as intended.

Written by the indexing model from the issue text.