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/locales/en.json

@@ -667,6 +667,11 @@
   "color_strip.audio.viz.vu_meter": "VU Meter",
   "color_strip.audio.device": "Audio Device:",
   "color_strip.audio.device.hint": "Audio input source. Loopback devices capture system audio output; input devices capture microphone or line-in.",
+  "color_strip.audio.channel": "Channel:",
+  "color_strip.audio.channel.hint": "Select which audio channel to visualize. Use Left/Right for stereo setups.",
+  "color_strip.audio.channel.mono": "Mono (L+R mix)",
+  "color_strip.audio.channel.left": "Left",
+  "color_strip.audio.channel.right": "Right",
   "color_strip.audio.sensitivity": "Sensitivity:",
   "color_strip.audio.sensitivity.hint": "Gain multiplier for audio levels. Higher values make LEDs react to quieter sounds.",
   "color_strip.audio.smoothing": "Smoothing:",