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/scripts/autoloads/session_manager.gd

@@ -30,6 +30,7 @@ func new_game() -> String:
 	if err != OK:
 		push_error("SessionManager: failed to create save dir %s: %s" % [
 			save_path, error_string(err)])
+		return ""
 	GameState.current_game_id = game_id
 	return game_id
 
@@ -74,7 +75,6 @@ func quit_to_menu(node: Node) -> void:
 	if _quit_dialog != null and is_instance_valid(_quit_dialog):
 		return  # Dialog already open
 	_quit_dialog = ConfirmationDialog.new()
-	_quit_dialog.title = ""
 	_quit_dialog.dialog_text = UIStrings.get_text("menu.confirm_quit")
 	_quit_dialog.ok_button_text = UIStrings.get_text("menu.confirm_yes")
 	_quit_dialog.cancel_button_text = UIStrings.get_text("menu.confirm_no")