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/routes/system.py

@@ -57,7 +57,7 @@ from wled_controller.api.schemas.system import (
     VersionResponse,
 )
 from wled_controller.core.backup.auto_backup import AutoBackupEngine
-from wled_controller.config import get_config
+from wled_controller.config import get_config, is_demo_mode
 from wled_controller.core.capture.screen_capture import get_available_displays
 from wled_controller.utils import atomic_write_json, get_logger
 
@@ -119,6 +119,7 @@ async def health_check():
         status="healthy",
         timestamp=datetime.now(timezone.utc),
         version=__version__,
+        demo_mode=get_config().demo,
     )
 
 
@@ -134,6 +135,7 @@ async def get_version():
         version=__version__,
         python_version=f"{sys.version_info.major}.{sys.version_info.minor}.{sys.version_info.micro}",
         api_version="v1",
+        demo_mode=get_config().demo,
     )
 
 
@@ -176,11 +178,20 @@ async def get_displays(
     logger.info(f"Listing available displays (engine_type={engine_type})")
 
     try:
-        if engine_type:
-            from wled_controller.core.capture_engines import EngineRegistry
+        from wled_controller.core.capture_engines import EngineRegistry
 
+        if engine_type:
             engine_cls = EngineRegistry.get_engine(engine_type)
             display_dataclasses = await asyncio.to_thread(engine_cls.get_available_displays)
+        elif is_demo_mode():
+            # In demo mode, use the best available engine (demo engine at priority 1000)
+            # instead of the mss-based real display detection
+            best = EngineRegistry.get_best_available_engine()
+            if best:
+                engine_cls = EngineRegistry.get_engine(best)
+                display_dataclasses = await asyncio.to_thread(engine_cls.get_available_displays)
+            else:
+                display_dataclasses = await asyncio.to_thread(get_available_displays)
         else:
             display_dataclasses = await asyncio.to_thread(get_available_displays)