Add thruster sound effect and integrate it with the player

- Added new `thruster.mp3` asset and loaded it in **BootScene** (`sfx_thruster` key).
- Extended `Player` class:
  * Created `thrusterSound` (looping) on construction.
  * Play sound while thrust is active and stop when thrust ends.
  * Ensure sound stops on player death and is destroyed on cleanup.
- Minor cleanup to stop thruster audio when the player entity is destroyed.

js/entities/Player.js

@@ -32,6 +32,8 @@ export default class Player {
         this.lastFired = 0;
         this.invincible = false;
         this.invincibleUntil = 0;
+
+        this.thrusterSound = scene.sound.add('sfx_thruster', { loop: true, volume: 0.6 });
     }
 
     update(time, delta) {
@@ -67,8 +69,10 @@ export default class Player {
             this.sprite.body.velocity.x += Math.cos(rad) * THRUST_ACCEL * dt;
             this.sprite.body.velocity.y += Math.sin(rad) * THRUST_ACCEL * dt;
             this.sprite.setTexture('player_thrust');
+            if (!this.thrusterSound.isPlaying) this.thrusterSound.play();
         } else {
             this.sprite.setTexture('player');
+            if (this.thrusterSound.isPlaying) this.thrusterSound.stop();
         }
 
         // Cap total speed
@@ -102,6 +106,7 @@ export default class Player {
     // Hide the ship and go invincible immediately – call before delaying respawn
     die() {
         this.alive = false;
+        if (this.thrusterSound.isPlaying) this.thrusterSound.stop();
         this.invincible = true;
         this.invincibleUntil = this.scene.time.now + 99999;
         this.sprite.body.setVelocity(0, 0);
@@ -173,6 +178,7 @@ export default class Player {
 
     destroy() {
         this.alive = false;
+        if (this.thrusterSound) { this.thrusterSound.stop(); this.thrusterSound.destroy(); }
         if (this.sprite && this.sprite.active) {
             this.sprite.destroy();
         }

js/scenes/BootScene.js

@@ -23,6 +23,7 @@ export default class BootScene extends Phaser.Scene {
         this.load.audio('sfx_missle',       'js/audio/fx/missle.mp3');
         this.load.audio('sfx_impact',       'js/audio/fx/impact.mp3');
         this.load.audio('sfx_alien_death',  'js/audio/fx/alien_death.mp3');
+        this.load.audio('sfx_thruster',     'js/audio/fx/thruster.mp3');
     }
 
     create() {