test: add Vitest frontend test runner

[kpassito]

Summary

• add Vitest with jsdom and Testing Library setup for frontend/unit tests
• add npm test and npm run test:watch scripts
• add a starter helper test for Stellar URL utilities
• run the new test command in CI and document local usage

Closes #125.

Verification

• npm test
• npm run lint
• npx tsc --noEmit
• npm run build