- Add `MessageState` trait defining UI reaction callbacks for generation lifecycle, model updates, provider status, resize, and tick events.
- Implement `App::handle_message` to route `AppMessage` variants to the provided `MessageState` and determine exit condition.
- Add `handler.rs` module with the trait and dispatch logic; re-export `MessageState` in `app/mod.rs`.
- Extend `ActiveGeneration` with a public `request_id` getter and clean up dead code annotations.
- Implement empty `MessageState` for `ChatApp` to integrate UI handling.
- Add `log` crate dependency for warning messages.