feat: admin mode, worlds 4-6, and stereo output fix

- Admin panel: add/remove stars, unlock worlds, reset progress (🛠 button)
- World 4 "Modulación" (8 levels): vibrato, sirena, wah-wah, auto-pan, FM, wobble bass
- World 5 "Efectos" (8 levels): delay, slapback, reverb, distortion, dub echo, shoegaze, ambient
- World 6 "Diseño Sonoro" (8 levels): kick, hi-hat, snare, pad, reese bass, laser, trance arp, final boss
- Star unlock progression: W4=36★, W5=48★, W6=60★ (total 48 levels, 144 stars)
- Fix stereo output: left/right channels now route through Tone.Merge for true stereo separation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/engine/audioEngine.js

@@ -136,13 +136,28 @@ function createNode(mod) {
       };
     }
     case 'output': {
-      const gain = new Tone.Gain(Tone.dbToGain(p.volume));
-      gain.toDestination();
+      // True stereo output: separate left/right channels → merge → master gain → destination
+      const leftGain = new Tone.Gain(1);
+      const rightGain = new Tone.Gain(1);
+      const merge = new Tone.Merge();
+      const master = new Tone.Gain(Tone.dbToGain(p.volume));
+      leftGain.connect(merge, 0, 0);
+      rightGain.connect(merge, 0, 1);
+      merge.connect(master);
+      master.toDestination();
       return {
-        node: gain,
-        inputs: { left: gain, right: gain },
+        node: master,
+        _merge: merge,
+        _leftGain: leftGain,
+        _rightGain: rightGain,
+        inputs: { left: leftGain, right: rightGain },
         outputs: {},
-        dispose: () => { gain.disconnect(); gain.dispose(); },
+        dispose: () => {
+          leftGain.disconnect(); leftGain.dispose();
+          rightGain.disconnect(); rightGain.dispose();
+          merge.disconnect(); merge.dispose();
+          master.disconnect(); master.dispose();
+        },
       };
     }
     case 'keyboard': {