Add disk selection dialog and improve GUI layout

- Add DiskSelectionDialog class for manual disk selection
- Implement select_installation_disk method with size display and model info
- Update GUI layout to place Network, Disk, and Timezone side by side in first row
- Add QScrollArea to prevent content from going out of sight
- Update start_installation to include selected disk in config
- Ensure proper validation for disk selection before installation
- Maintain auto-detection as default with manual override capability

This provides users with manual disk selection while keeping auto-detection
as the default, and ensures all GUI content remains visible with proper scrolling.