System media integration on three desktop OSes

Each desktop OS exposes a now-playing panel and hardware/media-key transport: macOS through its media-player framework's info centre and command centre, Windows through its system transport controls, Linux through the session-bus media interface. Put all three behind one facade in the player handler and let each platform choose its backend.

// adapted — Linux and Windows share one cross-platform backend, macOS has its own
private val systemControls =
    if (platform is Platform.Linux || platform is Platform.Windows) {
        runCatching { CrossPlatformNowPlaying(platform) }.getOrNull()
    } else {
        null
    }