Make auto-selected destination disk visible in AutoInstaller GUI

- Update detect_default_disk method to display selected disk in GUI label
- Show disk path and size when auto-detection finds suitable installation disk
- Users can now see which disk was automatically selected without interaction
- Maintains existing auto-detection logic while providing visual feedback
- Label format: 'Auto-selected: /dev/sda (32 GB)'

This ensures users are informed about the auto-selected installation disk
even when they don't manually interact with the disk selection dialog.