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/templates/modals/calibration.html

@@ -3,7 +3,8 @@
     <div class="modal-content" style="max-width: 700px;">
         <div class="modal-header">
             <h2 id="calibration-modal-title" data-i18n="calibration.title">📐 LED Calibration</h2>
-            <button class="tutorial-trigger-btn" onclick="startCalibrationTutorial()" data-i18n-title="calibration.tutorial.start" title="Start tutorial">?</button>
+            <button id="calibration-overlay-btn" class="tutorial-trigger-btn" onclick="toggleCalibrationOverlay()" data-i18n-title="overlay.button.show" title="Show overlay visualization" style="display:none">&#x1F4A1;</button>
+            <button id="calibration-tutorial-btn" class="tutorial-trigger-btn" onclick="startCalibrationTutorial()" data-i18n-title="calibration.tutorial.start" title="Start tutorial">?</button>
             <button class="modal-close-btn" onclick="closeCalibrationModal()" title="Close" data-i18n-aria-label="aria.close">&#x2715;</button>
         </div>
         <div class="modal-body">