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/main.py

@@ -49,6 +49,11 @@ logger = get_logger(__name__)
 # Get configuration
 config = get_config()
 
+# Seed demo data before stores are loaded (first-run only)
+if config.demo:
+    from wled_controller.core.demo_seed import seed_demo_data
+    seed_demo_data(config.storage)
+
 # Initialize storage and processing
 device_store = DeviceStore(config.storage.devices_file)
 template_store = TemplateStore(config.storage.templates_file)