feat: difficulty stars in emoji share

[prodzpod]

adds asterisks(one for hard, two for ultrahard) based on the difficulty the game is played in.

• ongoing exists to disable difficulty change once the game has started. this is technically redundancy but feels better than lifting the whole game logic up.
• currentDifficulty exists to preserve state where user solves the puzzle, changes the difficulty, then shares the puzzle without restarting.