That part about standing there watching with no way to save it hits hard, because that is the real loss right there, not the photo you never got. I went through the same thing last fall and it finally pushed me to just keep a cheap power bank in my glove box at all times, the kind that costs like twenty bucks at any store. The thing people miss is that it does not even have to be a big fancy setup, a small battery and a short cable will save you from that 2 percent death every single time. I also started carrying an old point and shoot camera I got at a thrift shop for almost nothing, and honestly it takes better shots of clouds than my phone ever did. Not saying you need to become a camera person, but having some backup plan means you stop missing the good ones. Mammatus clouds over Boise at golden hour sounds unreal and I am sorry you had to just watch it disappear.