A cleaner media library, with real video previews

Videos in the media library now show their first frame as a poster, in both grid and list view, with a play mark — instead of a colored placeholder. The same poster appears on the asset page's player.

The library itself follows the Catalog layout: folders and trash in a rail on the right, the same command bar and tables used across the module, and linked products that show their cover and title and open straight to the product page.