Add audio deps; extract input handling to threads

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`.

src/app/mod.rs

@@ -4,11 +4,12 @@ pub mod keybind;
 pub mod keybindings;
 pub mod state;
 pub mod states;
+pub mod threads;
 pub mod view;
 pub mod views;
 pub mod widgets;
 
-pub use app::{App, AppEvent, handle_input_events};
+pub use app::App;
 pub use keybind::handle_keybindings;
 pub use state::GameStates;
 pub use view::View;