Add LEDDeviceProvider abstraction and standby capability flag

Consolidate all device-type-specific logic into LEDDeviceProvider ABC
with provider registry. WLEDDeviceProvider handles client creation,
health checks, validation, mDNS discovery, and brightness control.
Routes now delegate to providers instead of using if/else type checks.

Add standby_required capability and expose device capabilities in API.
Target editor conditionally shows standby interval based on selected
device's capabilities.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/api/schemas/devices.py

@@ -85,6 +85,7 @@ class DeviceResponse(BaseModel):
     device_type: str = Field(default="wled", description="LED device type")
     led_count: int = Field(description="Total number of LEDs")
     enabled: bool = Field(description="Whether device is enabled")
+    capabilities: List[str] = Field(default_factory=list, description="Device type capabilities")
     calibration: Optional[Calibration] = Field(None, description="Calibration configuration")
     created_at: datetime = Field(description="Creation timestamp")
     updated_at: datetime = Field(description="Last update timestamp")