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/threads/mod.rs

@@ -0,0 +1,5 @@
+pub mod events;
+pub mod handle_input_events;
+
+pub use events::AppEvent;
+pub use handle_input_events::handle_input_events;