fix(client): address PR #67 review — error propagation, test cleanup

SessionManager.new_game() now returns "" on dir creation failure
instead of proceeding with a broken game-id. Main menu guards
against empty return. Test suite tracks and cleans up created
save directories in after_test().

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/ui/main_menu.gd

@@ -32,7 +32,10 @@ func _refresh_continue_state() -> void:
 
 
 func _on_new_game() -> void:
-	SessionManager.new_game()
+	var game_id := SessionManager.new_game()
+	if game_id.is_empty():
+		push_error("MainMenu: new_game() failed to create save directory — cannot start")
+		return
 	get_tree().change_scene_to_file(GAME_SCENE)