Add audio visualizer with spectrogram, beat-reactive art, and device selection

- New audio_analyzer service: loopback capture via soundcard + numpy FFT
- Real-time spectrogram bars below album art with accent color gradient
- Album art and vinyl pulse to bass energy beats
- WebSocket subscriber pattern for opt-in audio data streaming
- Audio device selection in Settings tab with auto-detect fallback
- Optimized FFT pipeline: vectorized cumsum bin grouping, pre-serialized JSON broadcast
- Visualizer config: enabled/fps/bins/device in config.yaml
- Optional deps: soundcard + numpy (graceful degradation if missing)

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

pyproject.toml

@@ -43,6 +43,10 @@ windows = [
     "screen-brightness-control>=0.20.0",
     "monitorcontrol>=3.0.0",
 ]
+visualizer = [
+    "soundcard>=0.4.0",
+    "numpy>=1.24.0",
+]
 dev = [
     "pytest>=7.0",
     "pytest-asyncio>=0.21",