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/locales/en.json

@@ -862,6 +862,9 @@
   "value_source.mode.rms": "RMS (Volume)",
   "value_source.mode.peak": "Peak",
   "value_source.mode.beat": "Beat",
+  "value_source.auto_gain": "Auto Gain:",
+  "value_source.auto_gain.hint": "Automatically normalize audio levels so output uses the full range, regardless of input volume",
+  "value_source.auto_gain.enable": "Enable auto-gain",
   "value_source.sensitivity": "Sensitivity:",
   "value_source.sensitivity.hint": "Gain multiplier for the audio signal (higher = more reactive)",
   "value_source.smoothing": "Smoothing:",
@@ -893,6 +896,13 @@
   "value_source.deleted": "Value source deleted",
   "value_source.delete.confirm": "Are you sure you want to delete this value source?",
   "value_source.error.name_required": "Please enter a name",
+  "value_source.test": "Test",
+  "value_source.test.title": "Test Value Source",
+  "value_source.test.connecting": "Connecting...",
+  "value_source.test.error": "Failed to connect",
+  "value_source.test.current": "Current",
+  "value_source.test.min": "Min",
+  "value_source.test.max": "Max",
   "targets.brightness_vs": "Brightness Source:",
   "targets.brightness_vs.hint": "Optional value source that dynamically controls brightness each frame (overrides device brightness)",
   "targets.brightness_vs.none": "None (device brightness)",