Add audio channel selection (mono/left/right), show device LED count in target editor

Audio capture now produces per-channel FFT spectrum and RMS alongside
the existing mono mix. Each audio color strip source can select which
channel to visualize via a new "Channel" dropdown. This enables stereo
setups with separate left/right segments on the same LED strip.

Also shows the device LED count under the device selector in the target
editor for quick reference.

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

server/src/wled_controller/templates/modals/target-editor.html

@@ -21,6 +21,7 @@
                     </div>
                     <small class="input-hint" style="display:none" data-i18n="targets.device.hint">Select the LED device to send data to</small>
                     <select id="target-editor-device"></select>
+                    <small id="target-editor-device-info" class="device-led-info" style="display:none"></small>
                 </div>
 
                 <div class="form-group" id="target-editor-segments-group">