I'm pretty sure the whole thing is built on the JVM, but I agree with you that having a peer run inside a js browser app via clojurescript would be a logical next step. (and arguably really useful)
Is Datomic just for JVM languages? At the moment, yes. We have ideas for how to enable Datomic on non-JVM languages while preserving as much of the embedded power as possible.
However, my big concern here would be security. You'd need to be able to supply a predicate for which datums are allowed to be synced to the client.