[Feature] Dynamic WPM mode

[Beethoven-n]

I notice that sometimes I struggle more with certain N-grams than others. I try to dial in and practice at a speed just faster than what I consider myself to be "struggling" at, and it seems to help! What might help more is if the WPM threshold was automatically adjustable based on your average WPM, maybe even with an offset of some sort.

A good way to implement this might be a flag, that when set, automatically adjusts the WPM threshold based on your average WPM. Pair that with an offset that the user can adjust, and you get the ability to target certain specific kinds of tests that you struggle with.

Give it a checkbox and another stepper, and you have everything on the frontend necessary to control these things.