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/static/css/modal.css

@@ -240,6 +240,13 @@
     background: var(--card-bg, #1e1e1e);
 }
 
+.device-led-info {
+    display: block;
+    margin-top: 4px;
+    color: var(--text-muted, #888);
+    font-size: 0.85em;
+}
+
 .segment-row-header {
     display: flex;
     justify-content: space-between;