There isn't a forum app and I've no intention of developing or providing one. The forum software is responsive which means it will adapt to whatever size browser view you have on your device, which is the simplest and cheapest set-up for me to maintain.
However, I
am looking at a service / app combination that allows you to get your CC forum alerts pushed to your mobile. Unfortunately though, it's not a free service and the charges are based on volume of alerts delivered. In the case of CC, that could be tens to hundreds of thousands per month, so it could be prohibitively expensive. I need to look into it and perhaps do a small-scale trial to see if the functionality is both useful and, of course, worth paying for.
It's on my to-do list ...