Add test preview for color strip sources with LED strip and rectangle views

New WebSocket endpoint streams real-time RGB frames from any CSS source.
Generic sources show a horizontal LED strip canvas. Picture sources show
a rectangle with per-edge canvases matching the calibration layout.

Server computes exact output indices per edge (offset + reverse + CW/CCW)
so the frontend renders edges in correct visual orientation.

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

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

@@ -928,6 +928,9 @@
   "color_strip.notification.test.ok": "通知已发送",
   "color_strip.notification.test.no_streams": "此源没有运行中的流",
   "color_strip.notification.test.error": "发送通知失败",
+  "color_strip.test.title": "预览测试",
+  "color_strip.test.connecting": "连接中...",
+  "color_strip.test.error": "无法连接到预览流",
   "color_strip.type.daylight": "日光循环",
   "color_strip.type.daylight.desc": "模拟24小时自然日光变化",
   "color_strip.type.daylight.hint": "模拟太阳在24小时内的色温变化——从温暖的日出到冷白的日光，再到温暖的日落和昏暗的夜晚。",