ci: add rolling macOS preview release workflow

[1lck]

Summary

• Add a daily scheduled macOS Preview build from preview/0.3.0.
• Publish a rolling GitHub Pre-release and replace its DMG assets on each build.
• Keep build artifacts for 14 days for troubleshooting.
• Avoid Homebrew Cask updates and formal in-app update discovery.

Validation

• YAML parsed successfully locally.
• Only .github/workflows/release-preview-macos.yml is included in this branch.

Activation

• The schedule becomes active after this workflow is merged into the default branch.