refactor: restructure game logic and state management into separate modules

- Moved game logic from `game.rs` to `game/mod.rs` and `game/state.rs` for better organization.
- Updated `App` to utilize the new `Game` struct and its state management.
- Refactored error handling
- Removed unused audio subsystem references

src/platform/emscripten.rs

@@ -3,8 +3,9 @@
 use std::borrow::Cow;
 use std::time::Duration;
 
-use crate::asset::{Asset, AssetError};
-use crate::platform::{Platform, PlatformError};
+use crate::asset::Asset;
+use crate::error::{AssetError, PlatformError};
+use crate::platform::Platform;
 
 /// Emscripten platform implementation.
 pub struct EmscriptenPlatform;