Add Sparkle update workflow

[progresshans]

Summary

• Add in-app update support with stable and alpha channels.
• Add release automation for publishing signed app builds and update feeds.
• Publish update feeds through GitHub Pages Actions.
• Document the release and update-feed workflow.

Validation

• swift test
• Workflow YAML parsing
• Packaging script syntax checks
• Metadata and appcast XML validation
• git diff --check

Notes

This is a draft PR so the release flow can be reviewed and verified before merging.

[progresshans]

Superseded. Replacing this draft with the original update branch.