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>

media_server/static/locales/en.json

@@ -23,6 +23,7 @@
   "player.source": "Source:",
   "player.unknown_source": "Unknown",
   "player.vinyl": "Vinyl mode",
+  "player.visualizer": "Audio visualizer",
   "state.playing": "Playing",
   "state.paused": "Paused",
   "state.stopped": "Stopped",
@@ -123,6 +124,15 @@
   "settings.section.scripts": "Scripts",
   "settings.section.callbacks": "Callbacks",
   "settings.section.links": "Links",
+  "settings.section.audio": "Audio",
+  "settings.audio.description": "Select which audio output device to capture for the visualizer.",
+  "settings.audio.device": "Loopback Device",
+  "settings.audio.auto": "Auto-detect",
+  "settings.audio.status_active": "Capturing audio",
+  "settings.audio.status_available": "Available, not capturing",
+  "settings.audio.status_unavailable": "Unavailable",
+  "settings.audio.device_changed": "Audio device changed",
+  "settings.audio.device_change_failed": "Failed to change audio device",
   "quick_access.no_items": "No quick actions or links configured",
   "display.loading": "Loading monitors...",
   "display.error": "Failed to load monitors",