Progress Estimation Improvements:
- Changed re-estimation interval from 10 seconds to 5 seconds for more frequent updates
- Multiplied initial time estimation by 4 to make it more conservative and realistic
- Better accuracy for long-running copy operations with more frequent recalculations

GUI Responsiveness Fix:
- Added QApplication.processEvents() calls in the copy loop to keep GUI responsive
- Exit button and other UI elements now remain interactive during copy operations
- Prevents blocking calls that would freeze the interface during file copying

Technical Details:
- Re-estimation now occurs every 5 seconds instead of 10 seconds
- Initial estimation multiplied by 4 for more conservative time prediction
- GUI event processing integrated into copy loop to maintain responsiveness
- Progress text now shows '5-second estimation' instead of '10-second estimation'