feat: add Sparkle auto-update for in-app update notifications

[owieth]

Currently there is no in-app update mechanism — users must manually check GitHub Releases for new versions. For a broader audience, integrating Sparkle 2.x with a GitHub Releases appcast would significantly improve the update experience.

Proposed approach

• Add Sparkle 2.x as a Swift Package dependency
• Host an appcast XML feed (can be generated and uploaded automatically by the release workflow)
• Add update check on app launch (respecting user preferences)

Notes

• Hardened runtime is already enabled (required for Sparkle)
• Distribution is GitHub releases only (no Mac App Store)

[owieth]

README already correctly states macOS 15.0 (Sequoia) at line 46, and docs/index.html also shows macOS 15+. No change needed — closing.