Add default values for AutoInstaller GUI selections

- Set default timezone to Africa/Johannesburg
- Auto-detect and select first available WiFi network with strongest signal
- Prompt for WiFi password when network is selected
- Fallback to first available Ethernet interface if no WiFi found
- Set default IP method to DHCP for both WiFi and Ethernet
- Auto-detect first available installation disk (not USB, >4GB)
- Pre-populate GUI with detected defaults on startup
- Add set_defaults() method called during GUI initialization
- Implement WiFi scanning with signal strength sorting
- Add detect_default_network() and detect_default_disk() methods
- Use QInputDialog for WiFi password input

This provides sensible defaults for all GUI selections, making the
installation process more user-friendly by pre-selecting appropriate
options based on the detected hardware and network environment.