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/css/styles.css

@@ -628,8 +628,39 @@ h1 {
 }
 
 @keyframes vinylSpin {
-    from { transform: rotate(var(--vinyl-offset, 0deg)); }
-    to { transform: rotate(calc(var(--vinyl-offset, 0deg) + 360deg)); }
+    from { transform: rotate(var(--vinyl-offset, 0deg)) scale(var(--vinyl-scale, 1)); }
+    to { transform: rotate(calc(var(--vinyl-offset, 0deg) + 360deg)) scale(var(--vinyl-scale, 1)); }
+}
+
+/* Audio Spectrogram Visualization */
+.spectrogram-canvas {
+    position: absolute;
+    bottom: -4px;
+    left: 50%;
+    transform: translateX(-50%);
+    z-index: 2;
+    pointer-events: none;
+    opacity: 0;
+    transition: opacity 0.3s ease;
+    border-radius: 0 0 8px 8px;
+}
+
+.visualizer-active .spectrogram-canvas {
+    opacity: 1;
+}
+
+.visualizer-active #album-art {
+    transition: transform 0.08s ease-out;
+}
+
+.visualizer-active .album-art-glow {
+    transition: opacity 0.08s ease-out;
+}
+
+/* Adapt spectrogram for vinyl mode */
+.album-art-container.vinyl .spectrogram-canvas {
+    bottom: -10px;
+    border-radius: 0 0 50% 50%;
 }
 
 .track-info {
@@ -1087,6 +1118,74 @@ button:disabled {
     margin-bottom: 1rem;
 }
 
+.audio-device-selector {
+    display: flex;
+    flex-direction: column;
+    gap: 0.75rem;
+}
+
+.audio-device-selector label {
+    display: flex;
+    flex-direction: column;
+    gap: 0.375rem;
+}
+
+.audio-device-selector label span {
+    font-size: 0.8125rem;
+    font-weight: 500;
+    color: var(--text-secondary);
+}
+
+.audio-device-selector select {
+    padding: 0.5rem 0.625rem;
+    border-radius: 8px;
+    border: 1px solid var(--border);
+    background: var(--bg-tertiary);
+    color: var(--text-primary);
+    font-size: 0.875rem;
+    cursor: pointer;
+}
+
+.audio-device-status {
+    font-size: 0.75rem;
+    display: flex;
+    align-items: center;
+    gap: 0.375rem;
+}
+
+.audio-device-status::before {
+    content: '';
+    display: inline-block;
+    width: 6px;
+    height: 6px;
+    border-radius: 50%;
+}
+
+.audio-device-status.active {
+    color: var(--accent);
+}
+
+.audio-device-status.active::before {
+    background: var(--accent);
+}
+
+.audio-device-status.available {
+    color: var(--text-secondary);
+}
+
+.audio-device-status.available::before {
+    background: var(--text-muted);
+}
+
+.audio-device-status.unavailable {
+    color: var(--text-muted);
+}
+
+.audio-device-status.unavailable::before {
+    background: var(--text-muted);
+    opacity: 0.5;
+}
+
 /* Link card in Quick Access */
 .link-card {
     text-decoration: none;