Firstly, I personally wouldn't breed her until she's in better shape. Metabolic issues can affect the foal too, and pregnancy hormones make laminitis even more likely. I know you said you understand this, but it is also not great for them to lose too quickly, so consider the possibility it would be too soon to breed this spring.
I exercise her as much as I can, but unfortunately, I am a full-time student and so taking the time to giver her regular exercise is challenging,
Setting up a track system that she has to move around, rather than open conventional paddock is helpful if possible for you. Google 'paddock paradise' for info & ideas.
but I also want to make sure her calorie intake is high enough that she is able to stay warm.
As for calorie/amount, the basic formula is to work out what her ideal weight should be and feed her at 1.5% of that bodyweight daily. More roughage is good in winter if needed.
A grazing muzzle part time, if your parents are around to check/remove, &/or slow feeder for hay, to slow her intake, &/or soaking & draining hay before feeding, to leach out some sugars if it's too rich. Check out Equine Cushing's and Insulin Resistance Information
& Katy Watts | Safergrass.org
for more info.
She shouldn't really need extra calories during her pregnancy - 1st trimester is the most taxing on a mare, then all OK till lactation takes a lot out of them. But I would advise she is adequately supplemented to give her well balanced nutrition, well before she is in foal. Unfortunately salt/min licks give them very little of anything, so I'd be looking for a good nutritional supp. Feedxl.com is a great resource for working out what that may be.