fix: add SSR safety and fix memory leak (qa-requested)

- Add 'use client' directive to GlobalBackground component
- Fix SSR-unsafe document access in usePageVisibility hook
- Fix memory leak in useIntersectionObserver cleanup function

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

src/hooks/usePageVisibility.ts

@@ -5,7 +5,9 @@ import { useState, useEffect } from 'react';
  * @returns boolean - true when page is visible, false when hidden
  */
 export const usePageVisibility = (): boolean => {
-  const [isVisible, setIsVisible] = useState<boolean>(!document.hidden);
+  const [isVisible, setIsVisible] = useState<boolean>(() =>
+    typeof document !== 'undefined' ? !document.hidden : true
+  );
 
   useEffect(() => {
     const handleVisibilityChange = () => {