feat: add conversion history view with real-time updates

Features:
- Created ConversionHistory component showing recent conversions
- Displays input/output formats, file sizes, and timestamps
- Relative time display (e.g., "5 minutes ago", "2 hours ago")
- Clear all history functionality with confirmation
- Individual history item removal
- Real-time updates when new conversions complete
- Custom event system for same-page updates
- Storage event listener for cross-tab synchronization
- Empty state with helpful messaging
- Clean, organized card-based layout
- Responsive design with proper spacing

Technical improvements:
- Enhanced history storage to dispatch custom events
- History component auto-refreshes on new conversions
- Maintains up to 10 most recent conversions
- Integrated seamlessly into main page layout

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/page.tsx

@@ -1,6 +1,7 @@
 'use client';
 
 import { FileConverter } from '@/components/converter/FileConverter';
+import { ConversionHistory } from '@/components/converter/ConversionHistory';
 import { ThemeToggle } from '@/components/layout/ThemeToggle';
 import { ToastProvider } from '@/components/ui/Toast';
 
@@ -23,7 +24,10 @@ export default function Home() {
 
         {/* Main content */}
         <main className="container mx-auto px-4 py-8 md:py-16">
-          <FileConverter />
+          <div className="space-y-8">
+            <FileConverter />
+            <ConversionHistory />
+          </div>
         </main>
 
         {/* Footer */}