Here's an idea.
If you want a desktop login/create form, then just create desktop versions (Login.dfm and NewAccount.dfm). If the framework doesn't find a Loginm.dfm it'll show the Login.dfm on both desktop and mobile.
After successful login, the framework will automatically chose Main.dfm for desktop and Mainm.dfm for mobile, do here create both a desktop and a mobile version of Main.
Given you likely want different sets of forms/features/workflows between desktop and mobile, from Main you would only show "fPage" forms, and from Mainm you would only show "fmPage" forms. Here you're in complete control of whether you next display a mobile or desktop form.
As for the mobile Mainm, if using layouts doesn't allow for phone/tablet/landscape/portrait layouts, you can use Mainm as a container and within Mainm decide which form/control you want to load into it. Or in Mainm immediately show a phone vs tablet form modally (so that Mainm is never really visible, it's always another form that the user actually sees.
Perhaps create a simple proof-of-concept project (empty forms with just a label to identify it and a button to do the next action) to test out these ideas and see how it works.
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