Prepare PowerLens 0.9.1 with Sparkle updates

[progresshans]

Summary

This PR prepares PowerLens 0.9.1 by adding Sparkle-powered software update support and the release workflow needed to publish signed, notarized builds with Stable and Alpha update feeds.

What Changed

• Added Sparkle-based update checking from the app menu and Settings.
• Added Stable and Alpha update channel selection in Settings.
• Embedded Sparkle configuration into packaged PowerLens app bundles.
• Added GitHub Actions workflows for CI and release publishing.
• Added GitHub Pages appcast files for Stable and Alpha update feeds.
• Refactored local and release packaging into shared bundle-staging helpers.
• Added local Sparkle update test tooling.
• Added release package validation for app metadata, appcast output, and distribution artifacts.

Testing

• Ran unit tests with swift test.
• Added CI coverage for scripts, metadata, appcast XML, and ad-hoc package smoke builds.
• Verified the Sparkle update flow with an Alpha update prompt.
• Verified that generated appcasts include signed update enclosures.

Release Notes

PowerLens 0.9.1 adds built-in software update support. Users can manually check for updates and choose between Stable and Alpha update channels from Settings.