feat: add start menu with interactive button

- Add StartMenu as the default game state
- Create StartMenuUI and StartButton components
- Implement menu UI with BGLGA title and Start Game button
- Add button interaction system with hover effects
- Set up proper state transitions from menu to game
- Update TODO.md to mark task as completed

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/main.rs

@@ -18,6 +18,7 @@ use resources::{ // Added StageConfigurations
 };
 use game_state::{
     cleanup_game_entities, cleanup_game_over_ui, setup_game_over_ui, AppState,
+    setup_start_menu_ui, cleanup_start_menu_ui, start_menu_button_system,
 };
 use player::{
     check_player_enemy_collisions, manage_invincibility, move_player, player_shoot,
@@ -112,6 +113,9 @@ fn main() {
                 .run_if(in_state(AppState::Playing)),
         )
         // UI and state management systems
+        .add_systems(OnEnter(AppState::StartMenu), setup_start_menu_ui)
+        .add_systems(OnExit(AppState::StartMenu), cleanup_start_menu_ui)
+        .add_systems(Update, start_menu_button_system.run_if(in_state(AppState::StartMenu)))
         .add_systems(OnEnter(AppState::GameOver), setup_game_over_ui)
         .add_systems(OnExit(AppState::Playing), cleanup_game_entities)
         .add_systems(OnExit(AppState::GameOver), cleanup_game_over_ui)