Add Vitest setup and 3 basic unit tests for payment flow

[iarturo]

The frontend currently has zero tests. This issue is about setting
up Vitest and writing 3 basic unit tests for the payment flow.

Tasks:

• Install Vitest and @testing-library/react
• Write test for successful order creation
• Write test for failed payment (insufficient balance)
• Write test for wallet connection state

Acceptance criteria:

• npm test runs and passes
• Tests live in web/src/__tests__/