Fix unhandled rejection when cancelling MFA setup after enable

The MFA onboarding slide runs a cleanup on unmount that cancels the
pending MFA setup when setup is not complete. The completion flag lived
in a ref that a separate effect synced from state, so an unmount in the
same tick as enable could fire the cancel against an account that already
has MFA enabled. The backend answers HTTP 409 and the discarded promise
surfaced as an unhandled rejection in error tracking.

Set the completion ref directly when enable succeeds, and settle the
cancel promise so a 409 is a no-op rather than a floating rejection.
This mirrors the try/catch already used in AccountSection.

Generated-By: PostHog Desktop
Task-Id: ef5a2561-fb31-4999-bd01-f6614fd03dd3

frontend/editor/src/core/components/onboarding/slides/MFASetupSlide.tsx

@@ -73,16 +73,14 @@ function MFASetupContent({ onMfaSetupComplete }: MFASetupSlideProps) {
     }
   }, [t]);
 
-  useEffect(() => {
-    setupCompleteRef.current = setupComplete;
-  }, [setupComplete]);
-
   useEffect(() => {
     void fetchMfaSetup();
 
     return () => {
       if (!setupCompleteRef.current) {
-        void accountService.cancelMfaSetup();
+        accountService.cancelMfaSetup().catch(() => {
+          // No pending secret to clear once MFA is enabled; ignore the 409.
+        });
       }
     };
   }, [fetchMfaSetup]);
@@ -113,6 +111,7 @@ function MFASetupContent({ onMfaSetupComplete }: MFASetupSlideProps) {
         setSubmitting(true);
         setMfaError("");
         await accountService.enableMfa(mfaSetupCode.trim());
+        setupCompleteRef.current = true;
         setSetupComplete(true);
         onMfaSetupComplete?.();
       } catch (err) {