feat(music): add gameplay background music

assets/sound.go

@@ -7,6 +7,7 @@ import (
 const (
 	SFX_SHAPE_LOCKED             audio.SoundID = 0
 	SFX_ROW_CLEARED              audio.SoundID = 1
+	SFX_MUSIC                    audio.SoundID = 3
 	SFX_MENU_SELECT              audio.SoundID = 0
 	SFX_MENU_ENTER               audio.SoundID = 1
 	SFX_MENU_SOUND_VOLUME_SELECT audio.SoundID = 1

game/state/handlers/gameplay.go

@@ -54,9 +54,12 @@ func (gp *GamePlay) Enter() {
 	gp.nextShape = gp.shapeQueue.Next()
 	gp.SpawnShape()
 	gp.lineClearAnimation.Reset()
+
+	audio.PlayLooped(assets.SFX_MUSIC)
 }
 
 func (GamePlay) Exit() {
+	audio.Stop(assets.SFX_MUSIC)
 }
 
 func (gp *GamePlay) SpawnShape() {