Add mute button and original soundtrack toggle

- Mute button: small [M] button alongside minimap zoom controls, turns red when active; directly sets AudioEngine master volume to 0, restores on unmute; persists in settings.cfg
- Original Soundtrack: checkbox in Settings > Audio that controls whether custom original music tracks (file: prefix) are included in zone music rotation; when disabled, only WoW MPQ tracks play; persists in settings.cfg
- ZoneManager.getRandomMusic() now filters file: paths when OST is disabled, falling back to full list if zone has no non-file tracks

include/ui/game_screen.hpp

@@ -97,6 +97,7 @@ private:
     bool pendingMinimapRotate = false;
     bool pendingMinimapSquare = false;
     bool pendingSeparateBags = true;
+    bool pendingUseOriginalSoundtrack = true;
 
     // UI element transparency (0.0 = fully transparent, 1.0 = fully opaque)
     float uiOpacity_ = 0.65f;
@@ -104,6 +105,10 @@ private:
     bool minimapSquare_ = false;
     bool minimapSettingsApplied_ = false;
 
+    // Mute state: mute bypasses master volume without touching slider values
+    bool soundMuted_ = false;
+    float preMuteVolume_ = 1.0f;  // AudioEngine master volume before muting
+
     /**
      * Render player info window
      */