Default cover art
=================
When a video's thumbnail can't be fetched, the downloaded file gets this picture.

Shipped: default.png  (the YouTube Turbo Downloader logo on white, 1000x1000)

To use your own image, put it here named exactly one of:
    default.jpg    (checked first)
    default.png
Recommended: square, 1000x1000 px or larger, under 3 MB.
If neither file exists, the add-on icon (src/assets/icons/icon64.png) is used instead.
