Fix Adalight power toggle using cached idle client and tracked state

Serial devices now route power on/off through the cached idle client
instead of opening a new serial connection (which caused PermissionError).
Adds tracked power_on state to DeviceState since Adalight has no
hardware power query.

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

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

@@ -55,6 +55,8 @@ class DeviceState:
     # Calibration test mode (works independently of target processing)
     test_mode_active: bool = False
     test_mode_edges: Dict[str, Tuple[int, int, int]] = field(default_factory=dict)
+    # Tracked power state for serial devices (no hardware query)
+    power_on: bool = True
 
 
 class ProcessorManager: