Component tests with React Testing Library

[iexwr]

Context

The frontend has no tests; PRs can regress behavior unnoticed.

Acceptance criteria

• Add RTL tests for Navbar (connect button), Create (validation + submit), and Request (Open/Paid).
• Mock lib/stellar.ts and the API client.
• npm run test includes them; runs in CI.

Technical notes

• Use vitest + @testing-library/react (add as devDependency).

Testing

• New *.test.tsx files; aim for meaningful assertions, not snapshot-only.