IntegrationError: Invalid value for stripe.confirmPayment(): elements should have a mounted Payment Element or Express Checkout Element.

## Error

```
Invalid value for stripe.confirmPayment(): elements should have a mounted Payment Element or Express Checkout Element.
```

## Stack trace

```
/v3:1 in i_
/v3:1 in c_
/v3:1 in s_
/v3:1
../app/javascript/components/donations/stripe-form/useStripeForm.ts:169 in
```

## Tracking Details
- **Project**: exercism-js
- **Sentry**: [IntegrationError: Invalid value for stripe.confirmPayment(): elements should have a mounted Payment Element or Express Checkout Element. ](https://thalamus-ai.sentry.io/issues/146410997/)
- **Culprit**: i_(v3)
- **First seen:** Sep 11, 2026

Start with app/javascript/components/donations/stripe-form/useStripeForm.ts at line 169 and compare the failing call with the Sentry stack trace. Investigate the donation form's element mounting state and verify the error no longer occurs when payment confirmation is attempted.

Written by the indexing model from the issue text.