**UPDATE: Problem resolved. This thread can be closed.
But if anyone completely new to Phonegap (like me) ever experiences problems with your Phonegap Build app not launching a website (if you are trying to make a Phonegap app that launches a website), I strongly advise thoroughly reading the cordova-inappbrowser documentation and doing lots of trial/error with your config.xml and index.html.
Also: don't build an app that does this if you're targeting app stores -- chances are good your app will be rejected. Even if targeting enterprise where this isn't an obstacle, my suggestion is to never do this anyway (there are far too many pitfalls and issues with doing so, including huge security risks).