Add overlay toggle to calibration dialog, fix serial reconnect on edge test

Add a 💡 button in the calibration modal header (CSS mode only) that
toggles the LED overlay visualization. Auto-stops overlay on modal close
if started from the dialog. Checks and reflects current overlay status
on modal open.

Fix serial devices creating a new connection on every edge test toggle,
which triggered Arduino bootloader resets. Now reuses the cached idle
client for all device types.

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

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

@@ -641,32 +641,15 @@ class ProcessorManager:
         pixels = [(0, 0, 0)] * ds.led_count
         await self._send_pixels_to_device(device_id, pixels)
 
-    def _is_serial_device(self, device_id: str) -> bool:
-        """Check if a device uses a serial (COM) connection."""
-        ds = self._devices.get(device_id)
-        return ds is not None and ds.device_type not in ("wled",)
-
     async def _send_pixels_to_device(self, device_id: str, pixels) -> None:
-        """Send pixels to a device.
+        """Send pixels to a device via cached idle client.
 
-        Serial devices: temporary connection (open, send, close).
-        WLED devices: cached idle client.
+        Reuses a cached connection to avoid repeated serial reconnections
+        (which trigger Arduino bootloader reset on Adalight devices).
         """
-        ds = self._devices[device_id]
         try:
-            if self._is_serial_device(device_id):
-                client = create_led_client(
-                    ds.device_type, ds.device_url,
-                    led_count=ds.led_count, baud_rate=ds.baud_rate,
-                )
-                try:
-                    await client.connect()
-                    await client.send_pixels(pixels)
-                finally:
-                    await client.close()
-            else:
-                client = await self._get_idle_client(device_id)
-                await client.send_pixels(pixels)
+            client = await self._get_idle_client(device_id)
+            await client.send_pixels(pixels)
         except Exception as e:
             logger.error(f"Failed to send pixels to {device_id}: {e}")