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/api/schemas/system.py

@@ -12,6 +12,7 @@ class HealthResponse(BaseModel):
     status: Literal["healthy", "unhealthy"] = Field(description="Service health status")
     timestamp: datetime = Field(description="Current server time")
     version: str = Field(description="Application version")
+    demo_mode: bool = Field(default=False, description="Whether demo mode is active")
 
 
 class VersionResponse(BaseModel):
@@ -20,6 +21,7 @@ class VersionResponse(BaseModel):
     version: str = Field(description="Application version")
     python_version: str = Field(description="Python version")
     api_version: str = Field(description="API version")
+    demo_mode: bool = Field(default=False, description="Whether demo mode is active")
 
 
 class DisplayInfo(BaseModel):