Handle immutable desktop releases

[omer-second]

Fixes the desktop release asset upload path for repos with immutable releases enabled.

What changed:

• If no GitHub Release exists, keep creating the release with DMG assets in the initial create path.
• If a draft release exists, upload the DMG assets and publish it.
• If a published immutable release already exists, fail with a clear error instead of trying an impossible asset upload.

This prevents the confusing HTTP 422 immutable release failure and supports a safe draft-release flow.