Extract SerialDeviceProvider base class and power off serial devices on shutdown

Create SerialDeviceProvider as the common base for Adalight and AmbiLED
providers, replacing the misleading Adalight→AmbiLED inheritance chain.
Subclasses now only override device_type and create_client(). Also send
explicit black frames to all serial LED devices during server shutdown.

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

server/src/wled_controller/core/processing/processor_manager.py

@@ -658,6 +658,14 @@ class ProcessorManager:
         for device_id in self._devices:
             await self._restore_device_idle_state(device_id)
 
+        # Power off serial LED devices before closing connections
+        for device_id, ds in self._devices.items():
+            if ds.device_type != "wled":
+                try:
+                    await self._send_clear_pixels(device_id)
+                except Exception as e:
+                    logger.error(f"Failed to power off {device_id} on shutdown: {e}")
+
         # Close any cached idle LED clients
         for did in list(self._idle_clients):
             await self._close_idle_client(did)