7) Testing and documentation

[manelHouri]

• Add lightweight tests:
  • Repository tests with mocked ApiClient.
  • SyncService tests: enqueue + flush success/fail behavior.
• Add docs:
  • README section “How to run app with backend” (base URL config, login, sync behavior).
• Deliverables:
  • A short “Developer Guide” page (1–2 pages) explaining offline-first + sync.