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/zh.json

@@ -862,6 +862,9 @@
   "value_source.mode.rms": "RMS（音量）",
   "value_source.mode.peak": "峰值",
   "value_source.mode.beat": "节拍",
+  "value_source.auto_gain": "自动增益：",
+  "value_source.auto_gain.hint": "自动归一化音频电平，使输出使用完整范围，无论输入音量大小",
+  "value_source.auto_gain.enable": "启用自动增益",
   "value_source.sensitivity": "灵敏度：",
   "value_source.sensitivity.hint": "音频信号的增益倍数（越高反应越灵敏）",
   "value_source.smoothing": "平滑：",
@@ -893,6 +896,13 @@
   "value_source.deleted": "值源已删除",
   "value_source.delete.confirm": "确定要删除此值源吗？",
   "value_source.error.name_required": "请输入名称",
+  "value_source.test": "测试",
+  "value_source.test.title": "测试值源",
+  "value_source.test.connecting": "连接中...",
+  "value_source.test.error": "连接失败",
+  "value_source.test.current": "当前",
+  "value_source.test.min": "最小",
+  "value_source.test.max": "最大",
   "targets.brightness_vs": "亮度源：",
   "targets.brightness_vs.hint": "可选的值源，每帧动态控制亮度（覆盖设备亮度）",
   "targets.brightness_vs.none": "无（设备亮度）",