Apple Silicon startup crashes when release URLs resolve to None

[sh-rn]

Problem

On Apple Silicon, platform.machine() returns arm64, but Darwin release URLs are only mapped for x86_64. The Block-DX URL lookup returns None, then startup crashes when it is used as a path:

TypeError: expected str, bytes or os.PathLike object, not NoneType
... os.path.basename(blockdx_url)

Reproduction

On an Apple Silicon Mac (Mac OS 26.4.1 - 25E253), launch the app after completing the source install.

Expected

• Support appropriate arm64 artefacts, or fail early with a clear unsupported-platform message.
• Validate the release URL lookup before path operations and do not persist null release URLs.