Add status change notifications and latency metrics

[Houtx]

What changed

• Add optional macOS notifications when a service transitions offline or recovers.
• Compare validated snapshots so first load, repeated refreshes, and request failures do not notify.
• Add a menu-bar toggle and a direct link to macOS notification settings when permission is denied.
• Show the latest probe latency next to each service uptime percentage in the menu bar and desktop widget.
• Add transition tests for failures, recovery, newly appearing failed services, case-insensitive model matching, and unchanged snapshots.
• Bump the app and Widget bundle to 1.6.0 (8) with release notes.

Why

The widget already showed whether services were online, but a person had to keep watching it. Local macOS notifications make outages and recovery actionable without adding telemetry or a server. Response latency adds an early signal when a service remains available but degrades.

Validation

• ./scripts/test.sh
• ./scripts/build.sh with warnings treated as errors for arm64 and x86_64
• Live launch check of the desktop widget with real status data and latency metrics
• Universal 2 architecture checks for the app and Widget extension
• Deep code-signing verification and DMG checksum verification