Frontend unit tests for error decoder & pre-flight layer

[gbengaeben]

Category: Testing | Priority: High | Dependencies: none | Out of scope: browser/UI component tests (separate issue)

Description & Context: frontend/src/lib/stellar/errors.ts and preflight.ts turn opaque Soroban failures into user guidance, but the frontend has no test framework and no committed tests — they were only smoke-tested ad hoc. A regression silently ships cryptic HostErrors to users.

Acceptance Criteria / DoD:

• vitest added to frontend/package.json with a npm test script; CI (frontend.yml) runs it on every push and PR
• errors.test.ts covers: every contract code 1–20 → mapped message; each HOST_ERROR_PATTERNS category (balance, fee, trustline, TTL, auth, contract-missing); Freighter rejection; network error; WalletError/PreflightError pass-through; unknown error fallback
• preflight.test.ts covers: runPreflight returns ALL failures (not fail-fast); every assert* returns null on pass / message on fail; mocked Freighter (isConnected/isAllowed/getAddress) and Horizon/RPC responses
• npm test exits 0 in CI

How to verify: cd frontend && npm test — all green; break one mapping and confirm the suite fails.