Add live LED strip preview via WebSocket on target cards

Stream real-time LED colors from running WLED targets to the browser via
binary WebSocket (RGB bytes, throttled to ~15 fps). Toggle button on
target card opens a compact canvas strip that renders each frame using
ImageData. Cached last frame is re-rendered after card reconciliation to
prevent flicker during auto-refresh.

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

server/src/wled_controller/static/js/app.js

@@ -87,7 +87,7 @@ import {
     showTargetEditor, closeTargetEditorModal, forceCloseTargetEditorModal, saveTargetEditor,
     startTargetProcessing, stopTargetProcessing,
     startTargetOverlay, stopTargetOverlay, deleteTarget,
-    cloneTarget,
+    cloneTarget, toggleLedPreview,
 } from './features/targets.js';
 
 // Layer 5: color-strip sources
@@ -295,6 +295,7 @@ Object.assign(window, {
     stopTargetOverlay,
     deleteTarget,
     cloneTarget,
+    toggleLedPreview,
 
     // color-strip sources
     showCSSEditor,