[P1] ViewModels: StateFlow UiState + unidirectional data flow

[amaury901130]

Replace LiveData + state enums + NetworkState + mutable error var with one immutable UiState data class exposed as StateFlow, actions via onAction, one-off events handled explicitly.

Acceptance criteria

• No LiveData in ViewModels
• ViewModel tests use runTest + Turbine or StateFlow assertions

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