Improves user-agent management with configurable options

[dhruv1955]

Summary

Improves user-agent management by replacing hardcoded Firefox user-agent with modern, configurable system supporting custom user-agents and rotation.

Changes

• Modern User-Agents: Added 8 current browser user-agents (Chrome, Firefox, Safari)
• Custom Support: --user-agent argument for custom strings
• Rotation: --rotate-user-agent flag for random rotation
• Smart Logic: get_user_agent() function with proper fallback
• Backward Compatible: Default behavior unchanged

Benefits

• Better detection avoidance through user-agent rotation
• Modern, up-to-date user-agents by default
• Flexibility for custom use cases
• Improved reliability and compatibility

Testing

• ✅ Custom user-agent works correctly
• ✅ Rotation produces different user-agents
• ✅ No breaking changes
• ✅ Help output shows new options