Focus Sparkle update alerts

[yvette-carlisle]

Summary

• register Rsnap's Sparkle updater as the standard user-driver delegate for scheduled update prompts
• bring the standard Sparkle update alert to the foreground when Sparkle handles an available update
• initialize the updater after the status item is configured, without adding any Update Available menu state

Verification

• cargo make fmt-swift
• cargo make test-macos-native-host
• cargo make test-macos-native-host-stage
• scripts/smoke/sparkle-update-local.sh --self-check
• cargo make check-swift
• git diff --check