Architecture selection for ISOs

[NN708]

Once Vanilla-OS/live-iso#403 is merged, both AMD64 and ARM64 ISOs will be available. The download flow should be updated to support both architectures.

Proposed changes:

• Retrieve both ISOs from GitHub Actions artifacts (example: https://github.com/vanilla-arm/live-iso/actions/runs/24023946208), and make ISOs directly downloadable (download ISO directly istead of a zip file #230)
• Add a dialog box for architecture selection (similar to https://elementary.io), including ISO checksums for verification