Fix TypeScript build error for NodeJS.Timeout type

Use ReturnType<typeof setTimeout> instead of NodeJS.Timeout for
browser compatibility in Docker build environment.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/pages/LEDPage.tsx

@@ -74,7 +74,7 @@ export function LEDPage() {
   const [color3, setColor3] = useState('#0000ff')
 
   // Ref for debouncing color picker API calls
-  const colorDebounceRef = useRef<NodeJS.Timeout | null>(null)
+  const colorDebounceRef = useRef<ReturnType<typeof setTimeout> | null>(null)
 
   // Effect automation state
   const [idleEffect, setIdleEffect] = useState<EffectSettings | null>(null)