Mobile testing / set UserAgent

I'm struggling with emulating any mobile with chromy. We have a website, that shows a desktop or mobile version depending on a UserAgent, but I didn't find a clear explanation how and where this functionality can be added inside Backstop. Could you provide maybe some examples or a clear way where to add it, or maybe somehow using puppeteer?

Start by reviewing BackstopJS's existing mobile-testing configuration and Puppeteer integration to locate where a UserAgent could be set. Done means providing a clear, reproducible example for emulating a mobile device or documenting why a code change is required.

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