Stamina could follow a logistic curve instead of linear

[keyspace]

Tried this previously with UI code as-is, and it worked OK-ish - except for the "black border" fade-in getting too choppy then, due to the simplistic way it's coded. So, likely depends on issue #1.

Something similar suggested by @chris_cz on Steam Workshop:

I would just like to add slight gradual slow down bellow 50%