feat: add explosion animations on entity destruction

Spawn expanding/fading orange explosion effects when enemies or
the player are destroyed. Explosions scale from 15x15 to 50x50
over 0.4s while fading from full opacity to transparent, then
auto-despawn.

Integration points:
- Enemy killed by player bullet (bullet.rs)
- Player hit by enemy bullet (bullet.rs)
- Player collides with enemy (player.rs) - both explode
- Captured player released (player.rs)

Refs: GAL-44

src/player.rs

@@ -9,6 +9,7 @@ use crate::constants::{
 };
 use crate::game_state::AppState;
 use crate::resources::{PlayerLives, PlayerRespawnTimer};
+use crate::systems::spawn_explosion;
 
 // Helper to spawn player (used in setup and respawn)
 pub fn spawn_player_ship(commands: &mut Commands) {
@@ -116,6 +117,7 @@ pub fn handle_captured_player(
                         &mut respawn_timer,
                         &mut next_state,
                         entity,
+                        _player_pos,
                     );
                     continue; // Skip the rest of processing for this player
                 }
@@ -131,6 +133,7 @@ pub fn handle_captured_player(
                     &mut respawn_timer,
                     &mut next_state,
                     entity,
+                    transform.translation,
                 );
             }
         }
@@ -144,11 +147,15 @@ fn lose_life_and_respawn(
     respawn_timer: &mut ResMut<PlayerRespawnTimer>,
     next_state: &mut ResMut<NextState<AppState>>,
     player_entity: Entity,
+    player_position: Vec3,
 ) {
     // Lose a life
     lives.count = lives.count.saturating_sub(1);
     println!("Lives remaining: {}", lives.count);
 
+    // Spawn explosion at player position before destroying
+    spawn_explosion(commands, player_position);
+
     // Destroy player
     commands.entity(player_entity).despawn();
 
@@ -218,11 +225,13 @@ pub fn check_player_enemy_collisions(
 
             if distance < PLAYER_ENEMY_COLLISION_THRESHOLD {
                 println!("Player hit by enemy!");
+                spawn_explosion(&mut commands, enemy_transform.translation);
                 commands.entity(enemy_entity).despawn(); // Despawn enemy
 
                 lives.count = lives.count.saturating_sub(1); // Decrement lives safely
                 println!("Lives remaining: {}", lives.count);
 
+                spawn_explosion(&mut commands, player_transform.translation);
                 commands.entity(player_entity).despawn(); // Despawn player
 
                 if lives.count > 0 {