Logout Use Case

Tears down the current session: clears the cached token, drops in-memory user state, and (best-effort) tells the backend to invalidate the refresh token.

1. Controller calls LogoutUseCase().
2. Repository.logout() fires POST /auth/logout (fire-and-forget).
3. Local data source clears the token from secure storage.
4. Returns Either<AuthFailure, Unit>.

• Backend unreachable → still clear local token; never block the user from logging out.
• Local storage write fails → surface a warning but continue navigation to /login.

• lib/features/auth/domain/usecases/logout_usecase.dart

Logout must always succeed locally — the user expects to land on /login. Treat the network call as best-effort cleanup.