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>