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/game/state.rs

@@ -0,0 +1,135 @@
+use sdl2::{image::LoadTexture, pixels::Color, render::TextureCreator, video::WindowContext};
+use smallvec::SmallVec;
+
+use crate::{
+    asset::{get_asset_bytes, Asset},
+    audio::Audio,
+    constants::RAW_BOARD,
+    entity::{
+        collision::{Collidable, CollisionSystem, EntityId},
+        ghost::{Ghost, GhostType},
+        item::Item,
+        pacman::Pacman,
+    },
+    error::{GameError, GameResult, TextureError},
+    map::Map,
+    texture::{
+        sprite::{AtlasMapper, AtlasTile, SpriteAtlas},
+        text::TextTexture,
+    },
+};
+
+/// The `GameState` struct holds all the essential data for the game.
+///
+/// This includes the score, map, entities (Pac-Man, ghosts, items),
+/// collision system, and rendering resources. By centralizing the game's state,
+/// we can cleanly separate it from the game's logic, making it easier to manage
+/// and reason about.
+pub struct GameState {
+    pub score: u32,
+    pub map: Map,
+    pub pacman: Pacman,
+    pub ghosts: SmallVec<[Ghost; 4]>,
+    pub items: Vec<Item>,
+    pub debug_mode: bool,
+
+    // Collision system
+    pub(crate) collision_system: CollisionSystem,
+    pub(crate) pacman_id: EntityId,
+    pub(crate) ghost_ids: SmallVec<[EntityId; 4]>,
+    pub(crate) item_ids: Vec<EntityId>,
+
+    // Rendering resources
+    pub(crate) atlas: SpriteAtlas,
+    pub(crate) map_texture: AtlasTile,
+    pub(crate) text_texture: TextTexture,
+
+    // Audio
+    pub audio: Audio,
+}
+
+impl GameState {
+    /// Creates a new `GameState` by initializing all the game's data.
+    ///
+    /// This function sets up the map, Pac-Man, ghosts, items, collision system,
+    /// and all rendering resources required to start the game. It returns a `GameResult`
+    /// to handle any potential errors during initialization.
+    pub fn new(texture_creator: &'static TextureCreator<WindowContext>) -> GameResult<Self> {
+        let map = Map::new(RAW_BOARD)?;
+
+        let pacman_start_node = map.start_positions.pacman;
+
+        let atlas_bytes = get_asset_bytes(Asset::Atlas)?;
+        let atlas_texture = texture_creator.load_texture_bytes(&atlas_bytes).map_err(|e| {
+            if e.to_string().contains("format") || e.to_string().contains("unsupported") {
+                GameError::Texture(TextureError::InvalidFormat(format!("Unsupported texture format: {e}")))
+            } else {
+                GameError::Texture(TextureError::LoadFailed(e.to_string()))
+            }
+        })?;
+        let atlas_json = get_asset_bytes(Asset::AtlasJson)?;
+        let atlas_mapper: AtlasMapper = serde_json::from_slice(&atlas_json)?;
+        let atlas = SpriteAtlas::new(atlas_texture, atlas_mapper);
+
+        let mut map_texture = SpriteAtlas::get_tile(&atlas, "maze/full.png")
+            .ok_or_else(|| GameError::Texture(TextureError::AtlasTileNotFound("maze/full.png".to_string())))?;
+        map_texture.color = Some(Color::RGB(0x20, 0x20, 0xf9));
+
+        let text_texture = TextTexture::new(1.0);
+        let audio = Audio::new();
+        let pacman = Pacman::new(&map.graph, pacman_start_node, &atlas)?;
+
+        // Generate items (pellets and energizers)
+        let items = map.generate_items(&atlas)?;
+
+        // Initialize collision system
+        let mut collision_system = CollisionSystem::default();
+
+        // Register Pac-Man
+        let pacman_id = collision_system.register_entity(pacman.position());
+
+        // Register items
+        let mut item_ids = Vec::new();
+        for item in &items {
+            let item_id = collision_system.register_entity(item.position());
+            item_ids.push(item_id);
+        }
+
+        // Create and register ghosts
+        let ghosts = [GhostType::Blinky, GhostType::Pinky, GhostType::Inky, GhostType::Clyde]
+            .iter()
+            .zip(
+                [
+                    map.start_positions.blinky,
+                    map.start_positions.pinky,
+                    map.start_positions.inky,
+                    map.start_positions.clyde,
+                ]
+                .iter(),
+            )
+            .map(|(ghost_type, start_node)| Ghost::new(&map.graph, *start_node, *ghost_type, &atlas))
+            .collect::<GameResult<SmallVec<[_; 4]>>>()?;
+
+        let ghost_ids = ghosts
+            .iter()
+            .map(|ghost| collision_system.register_entity(ghost.position()))
+            .collect::<SmallVec<[_; 4]>>();
+
+        Ok(Self {
+            score: 0,
+            map,
+            pacman,
+            ghosts,
+            items,
+            debug_mode: false,
+            collision_system,
+            pacman_id,
+            ghost_ids,
+            item_ids,
+            map_texture,
+            text_texture,
+            audio,
+            atlas,
+        })
+    }
+}