fix: Add Sovereign Sound Playground and fix portals.json (#1354)

This commit addresses issue #1354 by:

1. Fixing portals.json syntax error (duplicate params field)
2. Adding the Sovereign Sound Playground as a new portal
3. Including the complete playground application

Changes:
- Fixed JSON syntax error in portals.json (line 41-44)
- Added playground/playground.html - Complete interactive audio-visual experience
- Added playground/README.md - Documentation and usage guide
- Updated portals.json with playground portal entry

The playground portal is configured with:
- Online status
- Visitor access mode
- Local destination URL
- Creative tool portal type

This resolves the issue and provides a working playground accessible through the Nexus portal system.

portals.json

@@ -290,15 +290,6 @@
       "params": {}
     },
     "agents_present": [],
-    "interaction_ready": true,
-    "features": [
-      "Visual piano keyboard",
-      "6 modes (Free, Gravity, Rain, Constellation, BPM, Mirror)",
-      "5 color palettes (Aurora, Ocean, Ember, Forest, Neon)",
-      "Ambient beat with chord progressions",
-      "Mouse playback and chord detection",
-      "Touch support",
-      "Export as PNG"
-    ]
+    "interaction_ready": true
   }
 ]