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/api/routes/color_strip_sources.py

@@ -83,6 +83,7 @@ def _css_to_response(source, overlay_active: bool = False) -> ColorStripSourceRe
         visualization_mode=getattr(source, "visualization_mode", None),
         audio_device_index=getattr(source, "audio_device_index", None),
         audio_loopback=getattr(source, "audio_loopback", None),
+        audio_channel=getattr(source, "audio_channel", None),
         sensitivity=getattr(source, "sensitivity", None),
         color_peak=getattr(source, "color_peak", None),
         overlay_active=overlay_active,
@@ -164,6 +165,7 @@ async def create_color_strip_source(
             visualization_mode=data.visualization_mode,
             audio_device_index=data.audio_device_index,
             audio_loopback=data.audio_loopback,
+            audio_channel=data.audio_channel,
             sensitivity=data.sensitivity,
             color_peak=data.color_peak,
         )
@@ -237,6 +239,7 @@ async def update_color_strip_source(
             visualization_mode=data.visualization_mode,
             audio_device_index=data.audio_device_index,
             audio_loopback=data.audio_loopback,
+            audio_channel=data.audio_channel,
             sensitivity=data.sensitivity,
             color_peak=data.color_peak,
         )