[Enhancement] Improve error messages in code challenges #18

Problem\nWhen a player's code throws a syntax error or runtime exception, the error message is the raw JS error (e.g. 'ReferenceError: x is not defined'). This is intimidating for beginners.\n\nSolution\nMap common error types to friendly messages:\n- ReferenceError → 'Did you spell that variable correctly?'\n- SyntaxError → 'Check for missing brackets, parentheses, or commas'\n- TypeError → 'Make sure you're using the right type of value'\n\nLabels: enhancement, good first issue