Unify event architecture between api and keystore

[emlun]

With #483, the api and LocalStorageKeystore modules now expose two very different ways of attaching event listeners to these services. This PR aims to unify both APIs under a shared architecture based on the EventTarget type available in the JS engine.

As part of that, this also introduces util.cleanupListeners to help streamline setting up useEffect cleanup handlers to remove stale event listeners. This helper function sets up an AbortController and provides its signal so that event listeners can be automatically removed using the signal option of addEventListener.