feat: add scrolling starfield background

Add 150 stars with randomized positions, sizes, speeds, and brightness.
Stars scroll downward with parallax depth effect and wrap around at
screen edges. Also fixes Bevy 0.13 API issues in game_state.rs
(KeyCode::R → KeyR, Input → ButtonInput).

src/game_state.rs

@@ -197,11 +197,10 @@ pub fn start_menu_button_system(
 }
 
 pub fn handle_restart_input(
-    mut keyboard_input: ResMut<Input<KeyCode>>,
+    keyboard_input: Res<ButtonInput<KeyCode>>,
     mut restart_resource: ResMut<RestartPressed>,
-    mut app_state: ResMut<NextState<AppState>>,
 ) {
-    if keyboard_input.just_pressed(KeyCode::R) {
+    if keyboard_input.just_pressed(KeyCode::KeyR) {
         restart_resource.pressed = true;
     }
 }