Add min brightness threshold to LED targets

New per-target property: when effective output brightness
(max pixel value × device/source brightness) falls below
the threshold, LEDs turn off completely. Useful for cutting
dim flicker in audio-reactive and ambient setups.

Threshold slider (0–254) in target editor, badge on card,
hot-swap to running processors, persisted in storage.

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

server/src/wled_controller/static/locales/zh.json

@@ -896,6 +896,8 @@
   "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）",
+  "targets.min_brightness_threshold": "最低亮度阈值：",
+  "targets.min_brightness_threshold.hint": "当有效输出亮度（像素亮度 × 设备/源亮度）低于此值时，LED完全关闭（0 = 禁用）",
 
   "search.open": "搜索 (Ctrl+K)",
   "search.placeholder": "搜索实体... (Ctrl+K)",