Function build_large_test_app_state 

pub fn build_large_test_app_state(
    direct_chat_count: u32,
    group_chat_count: u32,
    messages_in_current_chat: u32,
) -> AppState

Deterministic high-volume shell fixture for list, navigation, and message rendering tests. This is intentionally synthetic: relay-backed tests cover protocol behaviour, while this keeps UI/perf tests fast and reproducible.