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/js/app.js

@@ -125,6 +125,7 @@ import {
     onNotificationFilterModeChange,
     notificationAddAppColor, notificationRemoveAppColor,
     testNotification,
+    testColorStrip, closeTestCssSourceModal,
 } from './features/color-strips.js';
 
 // Layer 5: audio sources
@@ -404,6 +405,7 @@ Object.assign(window, {
     onNotificationFilterModeChange,
     notificationAddAppColor, notificationRemoveAppColor,
     testNotification,
+    testColorStrip, closeTestCssSourceModal,
 
     // audio sources
     showAudioSourceModal,