xr57p-20230819-0950

Fixed the bug where the game would hang if you flipped a mountain tile late game.  (identified by @broquaint).  Now you can clear the board!  (but i've also realized some min-max strategies that break the little 'balance' there is...

problem:

• if you flipped a mountain late game, it would hang trying to find a path to it.
• problem occured because the choice was allowed since it was not a mountain yet (when hovered it was 'undiscovered' but in range; when initially flipped it was primary to do pathfinding), but then became mountain and so then became inaccessable. pathfinding woudl hang trying to search every possible tile on the grid.

solution:

• probably would like to change the order of flipping and moving so this problem cannot be reached
• for now: added check in beginning of pathfinding function: if target is a mountain, then return false immediately b/c we know no path is possible to an endpoint that is not accessible.