feat: add elapsed timer, input padding, and message separators

[kitan23]

Summary

UI polish improvements for better user experience:

• Elapsed timer: Shows generation time with interrupt hint (e.g., 3s · esc to interrupt)
• Input padding: Added padding to input box for better readability
• Message separators: Subtle separator line between responses and new user input
• Interrupt message: Shows "Interrupted by the user" when pressing Esc
• Cleanup: Removed unused action_stop_generation method

Screenshots