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/config.py

@@ -73,12 +73,22 @@ class Config(BaseSettings):
         case_sensitive=False,
     )
 
+    demo: bool = False
+
     server: ServerConfig = Field(default_factory=ServerConfig)
     auth: AuthConfig = Field(default_factory=AuthConfig)
     storage: StorageConfig = Field(default_factory=StorageConfig)
     mqtt: MQTTConfig = Field(default_factory=MQTTConfig)
     logging: LoggingConfig = Field(default_factory=LoggingConfig)
 
+    def model_post_init(self, __context: object) -> None:
+        """Override storage paths when demo mode is active."""
+        if self.demo:
+            for field_name in self.storage.model_fields:
+                value = getattr(self.storage, field_name)
+                if isinstance(value, str) and value.startswith("data/"):
+                    setattr(self.storage, field_name, value.replace("data/", "data/demo/", 1))
+
     @classmethod
     def from_yaml(cls, config_path: str | Path) -> "Config":
         """Load configuration from YAML file.
@@ -104,8 +114,9 @@ class Config(BaseSettings):
 
         Tries to load from:
         1. Environment variable WLED_CONFIG_PATH
-        2. ./config/default_config.yaml
-        3. Default values
+        2. WLED_DEMO=true → ./config/demo_config.yaml (if it exists)
+        3. ./config/default_config.yaml
+        4. Default values
 
         Returns:
             Config instance
@@ -115,6 +126,12 @@ class Config(BaseSettings):
         if config_path:
             return cls.from_yaml(config_path)
 
+        # Demo mode: try dedicated demo config first
+        if os.getenv("WLED_DEMO", "").lower() in ("true", "1", "yes"):
+            demo_path = Path("config/demo_config.yaml")
+            if demo_path.exists():
+                return cls.from_yaml(demo_path)
+
         # Try default location
         default_path = Path("config/default_config.yaml")
         if default_path.exists():
@@ -149,3 +166,8 @@ def reload_config() -> Config:
     global config
     config = Config.load()
     return config
+
+
+def is_demo_mode() -> bool:
+    """Check whether the application is running in demo mode."""
+    return get_config().demo