chore: prepare desktop beta release

[MadsenDev]

Summary

• set the standalone desktop app version to 2.0.8-beta.1
• make studio-v* beta tags create GitHub prereleases automatically
• upload artifacts to an existing release when the tag/release was created through the GitHub UI
• record the beta preparation in the changelog

Why

The desktop build is release-candidate ready, but the current workflow always creates a stable release and fails if a release already exists. This makes the first beta safe whether the tag is pushed from Git or the prerelease is created in GitHub.

Release after merge

Create studio-v2.0.8-beta.1 targeting main and mark it as a prerelease. The Desktop workflow will build Linux, macOS, and Windows downloads and attach them to that release.

Validation

• verified all three updated files on the remote branch
• preserved the existing package matrix and bundled-CLI validation