Add ALSA and Rodio crates to Cargo.toml and lockfile to enable audio
playback.
Update Nix expressions to include `alsa-lib` as a build input.
Refactor input event handling into a new `app::threads` module:
- Define `AppEvent` enum and `handle_input_events` function there.
- Adjust imports in `app/app.rs` and `main.rs` accordingly.
Remove the now‑unused `handle_input_events` and `AppEvent`
definitions from
`app/app.rs`.
GarandPLG