feat: major update - Markdown Editor, CodeMirror upgrades, SEO improvements, tool cleanup

- Added new Markdown Editor with live preview, GFM support, PDF/HTML/DOCX export
- Upgraded all paste fields to CodeMirror with syntax highlighting and expand/collapse
- Enhanced Object Editor with advanced URL fetching and preview mode
- Improved export views with syntax highlighting in Table/Object editors
- Implemented SEO improvements (FAQ schema, breadcrumbs, internal linking)
- Added Related Tools recommendations component
- Created custom 404 page with tool suggestions
- Consolidated tools: removed JSON, Serialize, CSV-JSON (merged into main editors)
- Updated documentation and cleaned up redundant files
- Updated release notes with user-centric improvements

src/components/AdColumn.js

@@ -0,0 +1,32 @@
+import React from 'react';
+import AdBlock from './AdBlock';
+
+/**
+ * AdColumn Component - Desktop sidebar with 3 ad units
+ * Hidden on mobile/tablet, visible on desktop (1200px+)
+ * All ads are 300x250 (Medium Rectangle) to comply with Google AdSense policies
+ * - Ads must be fully viewable without scrolling
+ * - No scrollable containers allowed
+ */
+const AdColumn = ({ 
+  slot1 = 'REPLACE_WITH_SLOT_1', 
+  slot2 = 'REPLACE_WITH_SLOT_2', 
+  slot3 = 'REPLACE_WITH_SLOT_3' 
+}) => {
+  return (
+    <aside className="hidden xl:block w-[300px] flex-shrink-0">
+      <div className="fixed top-20 right-8 w-[300px] space-y-5">
+        {/* Ad 1: Medium Rectangle */}
+        <AdBlock slot={slot1} size="300x250" />
+        
+        {/* Ad 2: Medium Rectangle */}
+        <AdBlock slot={slot2} size="300x250" />
+        
+        {/* Ad 3: Medium Rectangle */}
+        <AdBlock slot={slot3} size="300x250" />
+      </div>
+    </aside>
+  );
+};
+
+export default AdColumn;