feat: navigation can cycle through root board position

[latenitecoding]

This fixes the navigation logic so that users can use the left and right navigation arrows to cycle through the root board position and then to the first or last move. I also fixed a few bugs in the code. There were a few lines that assumed that the saved chess file study data would have a rootFEN, but they won't if they already exist. The error reporting for PGN parsing should have said FEN/PGN parsing.

[latenitecoding]

I just noticed that you have a catch in the loadFile to add the rootFEN if it's not already there. You could still leave the guards I have in place just in case the rootFEN ever becomes an empty string.

[chrislicodes]

Hey @latenitecoding, can you please rebase trunk *again? Not sure where the conflicts are?

[latenitecoding]

@chrislicodes Done