Add Local Image Selection for Faster Flashing

New Feature:
Add option 0 to the distro selection menu, allowing users to select a local OS image by specifying a path to a .7z file.

How it works:

When prompted to select a Linux distribution, choose option 0 ("Use local OS image file (.7z)").
You will be asked to enter the full path to your .7z image file, e.g.:
/home/indra/Downloads/theofficialgman-ubuntu-unity-noble-5.1.2-2024-10-18.7z
The script will use your local image file instead of downloading from the internet.

Why this matters:
This feature mitigates slow download speeds from https://download.switchroot.org by letting users provide their own image files directly, making the flashing process much faster and more reliable for those who already have the image.