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/components/ui/CoordinateDisplay.tsx

@@ -19,7 +19,7 @@ export function CoordinateDisplay({
 }: CoordinateDisplayProps) {
   return (
     <motion.div
-      className="fixed bottom-4 left-4 sm:bottom-6 sm:left-6 z-50"
+      className="fixed bottom-4 left-4 sm:bottom-6 sm:left-6 z-50 max-w-[calc(100vw-120px)] sm:max-w-none"
       initial={{ opacity: 0, y: 30 }}
       animate={{ opacity: 1, y: 0 }}
       exit={{ opacity: 0, y: 30 }}