Does leaving an Edge 830 connected to my Mac USB port charging indefinitely ruins its battery?

Yes, you are damaging the battery.  These types of lithium batteries last longest when stored at around 50-60% charged.  If your ride isn't going to be that long, then don't fully charge it.  For example, I generally charge to about 85% and it gets burned down to around 65% during my ride.  Also don't charge until right before your ride, the longer it sits fully charged, the worse off it is for the battery.  This goes for any of the typical lithium batteries in your life, most notably your phone.  

Cranky K, thank you for your answer.

I should have also added that letting it sit around stored very low on charge is very damaging as well.  Also keep in mind that they have a slow "self discharge" when in storage.  My wife doesn't use her Edge 130 all winter so I have to remember to put it on a maintenance charge at least every other month to get it back up to 60%.