- Move command-specific logic out of main.rs into Server::handle_* methods
(parsing remains in main and handlers accept already-parsed input).
- Handlers now return Result and return Err("ERROR:...") instead of sending
error messages directly to clients.
- main.rs now logs handler errors to console and sends the error string to
the client (removed `continue` statements after error sends).
- Move end_game_check from main.rs into lib.rs for reuse.
- Update handler implementations to follow the new error-returning pattern.
