Add "Only Show Current Letter" and "Only Show Current and Next Letter" Display Options

[Caliph3ubaidUrRe7man]

Monkeytype already supports display options like:

• "Only show current word"
• "Only show current and next word"
• "Only show current and next two words"

I’d like to suggest adding two similar options at the letter level:

• Only show current letter
• Only show current and next letter

This would help users improve fine-grain focus and reaction timing, especially when working on accuracy drills or training for single-character input speed. It fits naturally into the existing progressive visibility system already implemented for words.

Let me know if this would be a welcome addition. I’d be happy to provide mockups or test if needed.