How to support user actually signs the message

[llllllluc]

Hi really like this tutorial, i think it really shows how easy it is to create a rollup, it's just like writing a web2 api server. One question I have is how to let users actually sign the message they send (e.g. user needs to connect metamask or keplr to sign a tx in order to send a message, or there could be some implicit flow like user automatically signs).

Currently in frontend, the sender is randomly generated. When the tx reaches sequencer, it's unsigned, then sequencer signs it with sequencer private key. Is this intentional or the standard flow of rollup app? I thought in order to make it more like a blockchain app, user (message sender) needs to sign as well, right?

If this messenger rollup is built with cosmos sdk, it would support user sign it, right?