[P1] Data layer: repositories, domain models and typed errors

[amaury901130]

Replace UserManager/SessionManager and Result<Data> with repositories returning domain models and a sealed DomainError. Separate network DTOs from domain models (no password in the domain User).

Acceptance criteria

• UI never sees DTOs or Retrofit types
• Repository unit tests with fakes

Reference: Android Base — Modernization Audit & Recommendations — section: Target architecture