fix: validate dates and draft manual journals

[ryanduguid]

Summary

• validate YYYY-MM-DD values as real UTC calendar dates instead of accepting JavaScript rollover
• parse complete Xero /Date(...)/ wrappers with signed milliseconds and optional signed display offsets, without applying the offset twice
• require finite, exact-cent, zero-balanced manual journal lines and conservatively restrict create/update status to DRAFT
• pass validated/defaulted file input to xero-node

Compatibility

This intentionally rejects malformed dates, sub-cent or imbalanced manual journals, and consequential create/update statuses that were previously accepted locally. Existing valid DRAFT journals remain supported. It is separate from the forthcoming mutation confirmation contract.

Validation

• npx vitest run test/lib/validators.test.ts test/lib/formatters.test.ts (92 passed)
• npm test (135 passed)
• npm run build (passed)
• git diff --check (passed)

All tests use local synthetic fixtures; no Xero API was called.