Add value source test modal, auto-gain, brightness always-show, shared value streams

- Add real-time value source test: WebSocket endpoint streams get_value() at
  ~20Hz, frontend renders scrolling time-series chart with min/max/current stats
- Add auto-gain for audio value sources: rolling peak normalization with slow
  decay, sensitivity range increased to 0.1-20.0
- Always show brightness overlay on LED preview when brightness source is set
- Refactor ValueStreamManager to shared ref-counted streams (value streams
  produce scalars, not LED-count-dependent, so sharing is correct)
- Simplify acquire/release API: remove consumer_id parameter since streams
  are no longer consumer-dependent

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

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

@@ -72,6 +72,51 @@
     font-size: 0.9em;
 }
 
+/* Value source test chart canvas */
+.vs-test-canvas {
+    display: block;
+    width: 100%;
+    height: 200px;
+    background: #111;
+    border-radius: 6px;
+}
+
+.vs-test-stats {
+    display: flex;
+    gap: 20px;
+    align-items: center;
+    padding: 10px 0 0;
+    font-family: monospace;
+}
+
+.vs-test-stat {
+    display: flex;
+    align-items: center;
+    gap: 6px;
+}
+
+.vs-test-stat-label {
+    color: var(--text-muted, #888);
+    font-size: 0.85em;
+}
+
+.vs-test-stat-value {
+    font-weight: 600;
+    min-width: 50px;
+}
+
+.vs-test-value-large {
+    font-size: 1.3em;
+    color: #4caf50;
+}
+
+.vs-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; }