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/index.css

@@ -630,3 +630,92 @@ html, body, #root {
   top: 12px;
   right: 220px;
 }
+
+/* ===== Admin Panel ===== */
+.gm-admin-btn {
+  background: var(--surface);
+  border: 1px solid var(--border);
+  color: var(--text2);
+  padding: 6px 10px;
+  border-radius: 6px;
+  cursor: pointer;
+  font-size: 16px;
+  transition: all 0.15s;
+}
+.gm-admin-btn:hover { background: var(--surface2); color: var(--text); }
+
+.admin-overlay {
+  position: fixed; inset: 0; z-index: 1000;
+  background: rgba(0,0,0,0.7);
+  display: flex; align-items: center; justify-content: center;
+  backdrop-filter: blur(4px);
+}
+.admin-panel {
+  background: var(--panel);
+  border: 1px solid var(--border);
+  border-radius: 12px;
+  width: 90%; max-width: 700px; max-height: 85vh;
+  overflow-y: auto;
+  padding: 20px;
+}
+.admin-header {
+  display: flex; align-items: center; gap: 12px;
+  margin-bottom: 16px; padding-bottom: 12px;
+  border-bottom: 1px solid var(--border);
+}
+.admin-header h2 { font-size: 18px; color: var(--yellow); flex: 1; }
+.admin-total { color: var(--yellow); font-size: 14px; font-weight: 600; }
+.admin-close {
+  background: none; border: none; color: var(--text2);
+  cursor: pointer; font-size: 18px; padding: 4px 8px;
+}
+.admin-close:hover { color: var(--text); }
+
+.admin-actions {
+  display: flex; gap: 8px; margin-bottom: 16px;
+}
+.admin-action-btn {
+  padding: 6px 14px; border-radius: 6px; border: 1px solid var(--border);
+  background: var(--surface); color: var(--text); cursor: pointer;
+  font-size: 12px; font-weight: 500; transition: all 0.15s;
+}
+.admin-action-btn.gold { border-color: var(--yellow); color: var(--yellow); }
+.admin-action-btn.gold:hover { background: var(--yellow); color: var(--bg); }
+.admin-action-btn.danger { border-color: var(--red); color: var(--red); }
+.admin-action-btn.danger:hover { background: var(--red); color: #fff; }
+
+.admin-world { margin-bottom: 16px; }
+.admin-world-header {
+  display: flex; align-items: center; gap: 8px;
+  padding: 8px 0; border-bottom: 1px solid var(--border);
+  margin-bottom: 6px;
+}
+.admin-world-icon { font-size: 18px; }
+.admin-world-name { flex: 1; font-size: 13px; font-weight: 600; color: var(--text); }
+.admin-world-stars { font-size: 12px; color: var(--yellow); }
+.admin-unlock-btn {
+  padding: 3px 10px; border-radius: 4px; border: 1px solid var(--green);
+  background: transparent; color: var(--green); cursor: pointer;
+  font-size: 11px; transition: all 0.15s;
+}
+.admin-unlock-btn:hover { background: var(--green); color: var(--bg); }
+
+.admin-levels { display: flex; flex-direction: column; gap: 2px; }
+.admin-level {
+  display: flex; align-items: center; gap: 8px;
+  padding: 4px 8px; border-radius: 4px;
+  transition: background 0.1s;
+}
+.admin-level:hover { background: var(--surface); }
+.admin-level-num { font-size: 11px; color: var(--text2); width: 24px; }
+.admin-level-name { flex: 1; font-size: 12px; color: var(--text); }
+.admin-star-btns { display: flex; gap: 3px; }
+.admin-star-btn {
+  padding: 2px 6px; border-radius: 3px; border: 1px solid var(--border);
+  background: transparent; cursor: pointer; font-size: 11px;
+  color: var(--text2); transition: all 0.1s;
+}
+.admin-star-btn:hover { border-color: var(--yellow); color: var(--yellow); }
+.admin-star-btn.active { background: var(--yellow); color: var(--bg); border-color: var(--yellow); }
+.admin-star-btn.zero { color: var(--red); }
+.admin-star-btn.zero:hover { border-color: var(--red); color: var(--red); }