Fix pixel persistence and improve mobile UX

- Fix pixel data storage to include user information (userId, username, timestamp)
- Enhance zoom controls to center properly without drift
- Improve mobile modal centering with flexbox layout
- Add dynamic backend URL detection for network access
- Fix CORS configuration for development mode
- Add mobile-optimized touch targets and safe area support

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

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

frontend/src/app/layout.tsx

@@ -15,6 +15,9 @@ export const metadata: Metadata = {
 export const viewport = {
   width: 'device-width',
   initialScale: 1,
+  maximumScale: 5,
+  userScalable: true,
+  viewportFit: 'cover',
 };
 
 export default function RootLayout({