Allow apps to embed Disperse

Requested here: https://bsky.app/profile/awarm.space/post/3mkq5pen7vc2b

Probably just need to make the ShareScreen embeddable + customizable (https://tangled.org/quillmatiq.com/disperse/blob/main/src/components/ShareScreen.svelte), but will talk to folks to see what they expect out of it.