Add demo mode: virtual hardware sandbox for testing without real devices

Demo mode provides a complete sandbox environment with:
- Virtual capture engine (radial rainbow test pattern on 3 displays)
- Virtual audio engine (synthetic music-like audio on 2 devices)
- Virtual LED device provider (strip/60, matrix/256, ring/24 LEDs)
- Isolated data directory (data/demo/) with auto-seeded sample entities
- Dedicated config (config/demo_config.yaml) with pre-configured API key
- Frontend indicator (DEMO badge + dismissible banner)
- Engine filtering (only demo engines visible in demo mode)
- Separate entry point: python -m wled_controller.demo (port 8081)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/wled_controller/demo.py

@@ -0,0 +1,31 @@
+"""Demo mode entry point.
+
+Usage:
+    python -m wled_controller.demo
+
+Starts the server in demo mode with virtual devices, engines, and seed data.
+Equivalent to setting WLED_DEMO=true before starting the regular server.
+"""
+
+import os
+import sys
+
+
+def main():
+    os.environ["WLED_DEMO"] = "true"
+
+    import uvicorn
+    from wled_controller.config import get_config
+
+    config = get_config()
+    uvicorn.run(
+        "wled_controller.main:app",
+        host=config.server.host,
+        port=config.server.port,
+        log_level=config.server.log_level.lower(),
+        reload=False,
+    )
+
+
+if __name__ == "__main__":
+    main()