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`.

default.nix

@@ -2,6 +2,7 @@
   lib,
   rustPlatform,
   pkg-config,
+  alsa-lib,
 }:
 rustPlatform.buildRustPackage {
   name = "war-in-tunnels";
@@ -9,7 +10,7 @@ rustPlatform.buildRustPackage {
   version = "0.1.0";
 
   src = ./.;
-  # buildInputs = [ ];
+  buildInputs = [alsa-lib];
   nativeBuildInputs = [pkg-config];
   cargoHash = lib.fakeHash;