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/game_state.rs

@@ -1,10 +1,11 @@
 use bevy::prelude::*;
-use crate::components::{Bullet, Enemy, GameOverUI}; // Import necessary components
+use crate::components::{Bullet, Enemy, GameOverUI, StartMenuUI, StartButton}; // Import necessary components
 
 // --- Game States ---
 #[derive(Clone, Eq, PartialEq, Debug, Hash, Default, States)]
 pub enum AppState {
     #[default]
+    StartMenu,
     Playing,
     GameOver,
 }
@@ -65,4 +66,100 @@ pub fn cleanup_game_entities(
     // for entity in player_query.iter() {
     //     commands.entity(entity).despawn();
     // }
+}
+
+// --- Start Menu UI ---
+
+pub fn setup_start_menu_ui(mut commands: Commands) {
+    println!("Entering StartMenu state. Setting up UI.");
+    
+    // Root UI container
+    commands
+        .spawn((
+            NodeBundle {
+                style: Style {
+                    width: Val::Percent(100.0),
+                    height: Val::Percent(100.0),
+                    align_items: AlignItems::Center,
+                    justify_content: JustifyContent::Center,
+                    flex_direction: FlexDirection::Column,
+                    ..default()
+                },
+                ..default()
+            },
+            StartMenuUI,
+        ))
+        .with_children(|parent| {
+            // Title
+            parent.spawn(TextBundle::from_section(
+                "BGLGA",
+                TextStyle {
+                    font_size: 120.0,
+                    color: Color::WHITE,
+                    ..default()
+                },
+            ).with_style(Style {
+                margin: UiRect::bottom(Val::Px(50.0)),
+                ..default()
+            }));
+            
+            // Start Game Button
+            parent
+                .spawn((
+                    ButtonBundle {
+                        style: Style {
+                            width: Val::Px(250.0),
+                            height: Val::Px(80.0),
+                            border: UiRect::all(Val::Px(2.0)),
+                            justify_content: JustifyContent::Center,
+                            align_items: AlignItems::Center,
+                            ..default()
+                        },
+                        border_color: BorderColor(Color::WHITE),
+                        background_color: BackgroundColor(Color::rgb(0.1, 0.1, 0.5)),
+                        ..default()
+                    },
+                    StartButton,
+                ))
+                .with_children(|parent| {
+                    parent.spawn(TextBundle::from_section(
+                        "Start Game",
+                        TextStyle {
+                            font_size: 40.0,
+                            color: Color::WHITE,
+                            ..default()
+                        },
+                    ));
+                });
+        });
+}
+
+pub fn cleanup_start_menu_ui(mut commands: Commands, query: Query<Entity, With<StartMenuUI>>) {
+    println!("Exiting StartMenu state. Cleaning up UI.");
+    for entity in query.iter() {
+        commands.entity(entity).despawn_recursive();
+    }
+}
+
+pub fn start_menu_button_system(
+    mut interaction_query: Query<
+        (&Interaction, &mut BackgroundColor),
+        (Changed<Interaction>, With<StartButton>),
+    >,
+    mut app_state: ResMut<NextState<AppState>>,
+) {
+    for (interaction, mut color) in &mut interaction_query {
+        match *interaction {
+            Interaction::Pressed => {
+                println!("Start button pressed! Transitioning to Playing state.");
+                app_state.set(AppState::Playing);
+            }
+            Interaction::Hovered => {
+                color.0 = Color::rgb(0.2, 0.2, 0.7);
+            }
+            Interaction::None => {
+                color.0 = Color::rgb(0.1, 0.1, 0.5);
+            }
+        }
+    }
 }