Ok, took a look (sorry for the delay). Remove the max-width on the .boxed-layout CSS class and apply a max-width and margin 0 auto within the pages themselves. Hope I understood correctly what you are trying to do.
I’ll say upfront, I am not the most experience developer, when comes to Quasar or reactive UIs. Though, I’d like to offer my views on your question.
I see four “realms of application” that Quasar can currently cover (with more coming, like PWAs ).
Native Mobile applications through Cordova
Native Desktop applications through Electron
Desktop Web applications through a normal web server
Mobile Web application through a normal web server
Where Quasar helps with a lot of the work and even logic for the “View” of all of these applications is it standardizes and componentizes standard UI elements across all of these application realms. In other words, Quasar offers the large benefit of code reuse!
However, depending on what the application should do and in what realm, you are still going to be building different applications to certain extents. The only real possible “cross realm” application you can build might be between Desktop Web and Mobile Web applications. Still, I’d venture to say, that can be questionable. For instance, I’d really not want to build a large website administration application for smart phones or rather, I’d not worry about making sure that administration system works 100% on smart phones. I’d only want it to work 100% properly as a Desktop Web app.
So, to answer your question, it’s the applications you need to develop, which will determine the layouts you need. If you want an application for Desktop Web and Mobile Web, then you’ll probably need only one layout. However, to make it progressive, i.e. adding features for Desktop that a mobile device won’t be able to use, you’d be talking a good bit of juggling of the view logic. If your application isn’t too involved, that might be fine. However, if your applications are very involved, you might be better off going with two separate applications and thus, two separate layouts.