pub async fn remove_wallpaper(chat_npub: &str) -> Result<(), String>Expand description
Remove the chat’s wallpaper, reverting both sides to the default theme.
Publishes a kind-30078 vector-wallpaper tombstone (no url tag) so the
recipient and our other devices clear it too (latest-write-wins by
created_at), then DELETEs our blob and wipes local STATE/DB.