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.",