[2/2] … nonfree software in the users' browser. Once they send private data to the server, there is hardly any limit in practice to where it will go. (There are some dis-services that explicit demand users enter sensitive private data - those pose various more complex problems.) I've considered using Signal, since the client program (what I would need to run) is free software. However, it requires having a portable phone, and that requirement conclusively excludes me.
@rms signal should work without a smartphone nowadays, you can use eg. a VoIP phone service to get a number and receive the registration message with some SIP client