- Implement _tftp_get() for downloading files from Zynq via TFTP
- Add TftpSubStepFrame widget for Step 4 progress display
- Refactor Step 4 into 5 sub-steps: Upload, Download Verify, CRC Check, Reboot, Boot Verify
- Fix SubStepFrame.reset() bug that caused collapse after execution
- Fix layout overlap: Step 4 sub-step and progress bar now use separate rows
- Improve ProgressIndicator appearance with better styling and spacing
- Fix UART monitor to start regardless of _uart_available flag
- Add CRC32 verification between uploaded and downloaded files
