Sorry, just to clarify: you want to embed Chachi with all its HTML and JS inside another client? Or do you want to write a different chat UI that can interact with the same chat rooms that Chachi does?
I want both. now which NIP do i use for these?
If you want to make a chat client you use NIP-29. There are other clients that do it, like https://chat29.pages.dev/groups.0xchat.com'zcfCdcfn7b27TYtw/ and https://groups.nip29.com and 0xchat etc -- am I misunderstanding you?
If you want to embed an HTML client somewhere else then there is not a NIP for it because it's not advisable at all, but you would have to use an <iframe> or some other way to render HTML in that other environment. And then you could inject a faux window.nostr global in there. Not the same thing, but as an inspiration, https://github.com/fiatjaf/window.nostr.js injects a window.nostr global in websites, but instead of an extension it powers that with NIP-46 calls to bunkers.