feat: menu BGM theme + stop chapter music on return to menu

Assets/Game/Data/Resources/Music/bgm_menu.wav.meta

@@ -0,0 +1,23 @@
+fileFormatVersion: 2
+guid: f362ca68de7f71d4e844eab4d0e79abf
+AudioImporter:
+  externalObjects: {}
+  serializedVersion: 8
+  defaultSettings:
+    serializedVersion: 2
+    loadType: 0
+    sampleRateSetting: 0
+    sampleRateOverride: 44100
+    compressionFormat: 1
+    quality: 1
+    conversionMode: 0
+    preloadAudioData: 0
+  platformSettingOverrides: {}
+  forceToMono: 0
+  normalize: 1
+  loadInBackground: 0
+  ambisonic: 0
+  3D: 1
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 

Assets/Game/Scripts/Mono/UI/MainMenuController.cs

@@ -2,6 +2,7 @@ using UnityEngine;
 using UnityEngine.UI;
 using TMPro;
 using AjedrezPurgatorio.Meta;
+using AjedrezPurgatorio.Audio;
 
 namespace AjedrezPurgatorio.UI
 {
@@ -56,6 +57,11 @@ namespace AjedrezPurgatorio.UI
         {
             UpdateContinueButtonVisibility();
             CreateAlternateCampaignButton();
+
+            // Tema propio del menú; además reemplaza el BGM de capítulo
+            // que sigue sonando al volver (AudioManager es persistente).
+            if (AudioManager.Instance != null)
+                AudioManager.Instance.PlayTheme("menu");
         }
 
         private void OnDestroy()