Introduce `App` with provider manager, unbounded message channel, and active generation tracking.
Add `AppMessage` enum covering UI events, generation lifecycle (start, chunk, complete, error), model refresh, and provider status updates.
Implement `start_generation` to spawn asynchronous generation tasks, stream results, handle errors, and abort any previous generation.
Expose the new module via `pub mod app` in the crate root.
vikingowl
ea04a25ed6