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/core/processing/audio_stream.py

@@ -68,6 +68,7 @@ class AudioColorStripStream(ColorStripStream):
         self._auto_size = not source.led_count
         self._led_count = source.led_count if source.led_count and source.led_count > 0 else 1
         self._mirror = bool(getattr(source, "mirror", False))
+        self._audio_channel = getattr(source, "audio_channel", "mono")  # mono | left | right
         with self._colors_lock:
             self._colors: Optional[np.ndarray] = None
 
@@ -193,6 +194,16 @@ class AudioColorStripStream(ColorStripStream):
                 elapsed = time.perf_counter() - loop_start
                 time.sleep(max(frame_time - elapsed, 0.001))
 
+    # ── Channel selection ─────────────────────────────────────────
+
+    def _pick_channel(self, analysis):
+        """Return (spectrum, rms) for the configured audio channel."""
+        if self._audio_channel == "left":
+            return analysis.left_spectrum, analysis.left_rms
+        elif self._audio_channel == "right":
+            return analysis.right_spectrum, analysis.right_rms
+        return analysis.spectrum, analysis.rms
+
     # ── Spectrum Analyzer ──────────────────────────────────────────
 
     def _render_spectrum(self, buf: np.ndarray, n: int, analysis) -> None:
@@ -200,7 +211,7 @@ class AudioColorStripStream(ColorStripStream):
             buf[:] = 0
             return
 
-        spectrum = analysis.spectrum
+        spectrum, _ = self._pick_channel(analysis)
         sensitivity = self._sensitivity
         smoothing = self._smoothing
         lut = self._palette_lut
@@ -249,7 +260,8 @@ class AudioColorStripStream(ColorStripStream):
             buf[:] = 0
             return
 
-        rms = analysis.rms * self._sensitivity
+        _, ch_rms = self._pick_channel(analysis)
+        rms = ch_rms * self._sensitivity
         # Temporal smoothing on RMS
         rms = self._smoothing * self._prev_rms + (1.0 - self._smoothing) * rms
         self._prev_rms = rms