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/js/app.js

@@ -120,6 +120,7 @@ import {
     showValueSourceModal, closeValueSourceModal, saveValueSource,
     editValueSource, cloneValueSource, deleteValueSource, onValueSourceTypeChange,
     addSchedulePoint,
+    testValueSource, closeTestValueSourceModal,
 } from './features/value-sources.js';
 
 // Layer 5: calibration
@@ -360,6 +361,8 @@ Object.assign(window, {
     deleteValueSource,
     onValueSourceTypeChange,
     addSchedulePoint,
+    testValueSource,
+    closeTestValueSourceModal,
 
     // calibration
     showCalibration,