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`.

flake.nix

@@ -42,15 +42,16 @@
       develop = naerskLib.buildPackage {
         name = "war-in-tunnels";
         src = ./.;
-        # buildInputs = with pkgs; [];
+        buildInputs = with pkgs; [alsa-lib];
         nativeBuildInputs = with pkgs; [pkg-config];
       };
     };
 
     devShells.${system}.default = pkgs.mkShell {
       env.RUST_SRC_PATH = "${pkgs.rust.packages.stable.rustPlatform.rustLibSrc}";
-      buildInputs = [
+      buildInputs = with pkgs; [
         rustToolchain
+        alsa-lib
       ];
       nativeBuildInputs = with pkgs; [pkg-config];