feat: 增强注册流程，处理用户已存在错误，更新相关逻辑和测试用例

background/logging-status.js

@@ -91,6 +91,11 @@
       return /当前邮箱已存在，需要重新开始新一轮/.test(message);
     }
 
+    function isSignupUserAlreadyExistsFailure(error) {
+      const message = getErrorMessage(error);
+      return /SIGNUP_USER_ALREADY_EXISTS::|user_already_exists/i.test(message);
+    }
+
     function isStep9RecoverableAuthError(error) {
       const message = String(typeof error === 'string' ? error : error?.message || '');
       return /STEP9_OAUTH_RETRY::/i.test(message)
@@ -162,6 +167,7 @@
       hasSavedProgress,
       isLegacyStep9RecoverableAuthError,
       isRestartCurrentAttemptError,
+      isSignupUserAlreadyExistsFailure,
       isStep9RecoverableAuthError,
       isStepDoneStatus,
       isVerificationMailPollingError,