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/templates/index.html

@@ -127,6 +127,7 @@
     {% include 'modals/audio-template.html' %}
     {% include 'modals/test-audio-template.html' %}
     {% include 'modals/value-source-editor.html' %}
+    {% include 'modals/test-value-source.html' %}
 
     {% include 'partials/tutorial-overlay.html' %}
     {% include 'partials/image-lightbox.html' %}