Cache idle LED clients to avoid repeated Arduino resets

Adalight devices trigger a ~3s bootloader reset on every serial
connection open. Add a persistent idle client cache in ProcessorManager
so calibration test toggles, static color changes, and auto-restore
reuse an existing connection instead of creating a new one each time.

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

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

@@ -291,7 +291,7 @@ async def delete_device(
 
         # Remove from manager
         try:
-            manager.remove_device(device_id)
+            await manager.remove_device(device_id)
         except (ValueError, RuntimeError):
             pass
 
@@ -497,11 +497,7 @@ async def set_device_color(
     # If device is idle, apply the color immediately
     if color is not None and not manager.is_device_processing(device_id):
         try:
-            provider = get_provider(device.device_type)
-            await provider.set_color(
-                device.url, color,
-                led_count=device.led_count, baud_rate=device.baud_rate,
-            )
+            await manager.send_static_color(device_id, color)
         except Exception as e:
             logger.warning(f"Failed to apply static color immediately: {e}")