feat: allow custom thumbnail selection/upload for shared links

[004Ongoro]

Currently, when sharing a link on daily.dev, the platform automatically pulls the default Open Graph (OG) image from the source URL. Often, these images are low resolution, poorly cropped, or don't accurately represent the specific point I'm trying to highlight in the post. This can lead to lower click-through rates and less engagement from the community.
​
I would like the ability to change or customize the thumbnail image during the "Submit a Link" process. Specifically:
​* Image Picker: A way to cycle through other images found on the source page.
​* Manual Upload: An option to upload a custom JPG/PNG to serve as the post's cover image.
​Preview: A real-time preview of how the thumbnail will look in the feed before hitting "Post."

Currently, the only "workaround" is to accept the default image or hope the original website owner updates their metadata, which isn't possible for third-party content.

[idoshamun]

Hi @004Ongoro,

Thanks a lot for this suggestion! 🙏