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/static/js/app.js

@@ -120,7 +120,7 @@ import {
     showCalibration, closeCalibrationModal, forceCloseCalibrationModal, saveCalibration,
     updateOffsetSkipLock, updateCalibrationPreview,
     setStartPosition, toggleEdgeInputs, toggleDirection, toggleTestEdge,
-    showCSSCalibration,
+    showCSSCalibration, toggleCalibrationOverlay,
 } from './features/calibration.js';
 
 // Layer 6: tabs
@@ -345,6 +345,7 @@ Object.assign(window, {
     toggleDirection,
     toggleTestEdge,
     showCSSCalibration,
+    toggleCalibrationOverlay,
 
     // tabs
     switchTab,