As a MixPost Pro User, when I upload a video to a post, I want to select which still image or frame from my video will be used for the thumbnail on platforms supporting thumbnails wo that I can customise the look of my post and enhance the impact of my content.

Details:

Implement a Thumbnail Selector feature within MixPost Pro that allows users to choose a specific frame from their uploaded video to use as the thumbnail OR/AND to upload an image to be used as thumbnail.

Functionality:

• Provide an interface for users to preview and select a frame from their video as the thumbnail.
• Automatically apply the selected thumbnail to platforms that support this feature.
• Maintain default behaviour for platforms that do not support custom thumbnails.