Highlight and practice problematic letter combinations

[alexchexes]

It would be great to have the option to see problematic letter pairs or combinations (2 or maybe 3 letters) at the end of a typing session and be able to practice those combinations, similar to how we can currently practice difficult words after finishing.

This is a separate idea (to show weak letter combinations at the end), but it is related to and would greatly complement the other feature request - (link) to filter words by regex or letter sequences.