fix: clear TOTP secret in test for proper isolation

dashboard/backend/tests/unit/test_auth.py

@@ -204,6 +204,9 @@ class TestTOTP:
 
     def test_load_totp_secret_from_env(self, mock_config, temp_auth_file, monkeypatch):
         """Test loading TOTP secret from environment variable."""
+        # Clear any existing secret in file first
+        save_totp_secret("")
+
         monkeypatch.setenv("TOTP_SECRET", "ENV_SECRET_KEY")
         import importlib
         import config