Add real-time audio spectrum test for audio sources and templates

- Add WebSocket endpoints for live audio spectrum streaming at ~20Hz
- Audio source test: resolves device/channel, shares stream via ref-counting
- Audio template test: includes device picker dropdown for selecting input
- Canvas-based 64-band spectrum visualizer with falling peaks and beat flash
- Channel-aware: mono sources show left/right/mixed spectrum correctly

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

server/src/wled_controller/static/css/modal.css

@@ -18,6 +18,60 @@
     z-index: 2500;
 }
 
+/* Audio test spectrum canvas */
+.audio-test-canvas {
+    display: block;
+    width: 100%;
+    height: 200px;
+    background: #111;
+    border-radius: 6px;
+}
+
+.audio-test-stats {
+    display: flex;
+    gap: 20px;
+    align-items: center;
+    padding: 10px 0 0;
+    font-family: monospace;
+}
+
+.audio-test-stat {
+    display: flex;
+    align-items: center;
+    gap: 6px;
+}
+
+.audio-test-stat-label {
+    color: var(--text-muted, #888);
+    font-size: 0.85em;
+}
+
+.audio-test-stat-value {
+    font-weight: 600;
+    min-width: 50px;
+}
+
+.audio-test-beat-dot {
+    display: inline-block;
+    width: 10px;
+    height: 10px;
+    border-radius: 50%;
+    background: #444;
+    transition: background 0.1s;
+}
+
+.audio-test-beat-dot.active {
+    background: #ff4444;
+    box-shadow: 0 0 8px #ff4444;
+}
+
+.audio-test-status {
+    text-align: center;
+    padding: 8px 0;
+    color: var(--text-muted, #888);
+    font-size: 0.9em;
+}
+
 @keyframes fadeIn {
     from { opacity: 0; }
     to { opacity: 1; }