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/en.json

@@ -928,6 +928,9 @@
   "color_strip.notification.test.ok": "Notification sent",
   "color_strip.notification.test.no_streams": "No running streams for this source",
   "color_strip.notification.test.error": "Failed to send notification",
+  "color_strip.test.title": "Test Preview",
+  "color_strip.test.connecting": "Connecting...",
+  "color_strip.test.error": "Failed to connect to preview stream",
   "color_strip.type.daylight": "Daylight Cycle",
   "color_strip.type.daylight.desc": "Simulates natural daylight over 24 hours",
   "color_strip.type.daylight.hint": "Simulates the sun's color temperature throughout a 24-hour day/night cycle — from warm sunrise to cool daylight to warm sunset and dim night.",

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

@@ -928,6 +928,9 @@
   "color_strip.notification.test.ok": "Уведомление отправлено",
   "color_strip.notification.test.no_streams": "Нет запущенных потоков для этого источника",
   "color_strip.notification.test.error": "Не удалось отправить уведомление",
+  "color_strip.test.title": "Предпросмотр",
+  "color_strip.test.connecting": "Подключение...",
+  "color_strip.test.error": "Не удалось подключиться к потоку предпросмотра",
   "color_strip.type.daylight": "Дневной цикл",
   "color_strip.type.daylight.desc": "Имитация естественного дневного света за 24 часа",
   "color_strip.type.daylight.hint": "Имитирует цветовую температуру солнца в течение суток — от тёплого рассвета до прохладного дневного света, заката и ночи.",

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。",