feat(system-monitor): add CPU and memory usage gauges in notch header

[teszerrakt]

Summary

Adds an optional System Monitor feature that displays CPU and memory usage as circular gauge rings in the notch header area. Resolves #1082.

• Polls CPU and memory stats every 3 seconds using macOS Mach kernel APIs (zero new dependencies)
• Color-coded gauge rings: green/yellow/red based on configurable thresholds
• Click-to-show popover with detailed stats and "Open Activity Monitor" button
• Disabled by default — user opts in via Settings > System Monitor

Screenshots

System Monitor in Notch Header & Popover

Settings View

Changes

New Files

File	Description
boringNotch/managers/SystemMonitorManager.swift	Singleton manager polling CPU (host tick deltas) and memory (VM stats — active+wired+compressed) via Mach APIs. Cached host port with proper deallocation in deinit.
boringNotch/components/SystemMonitor/SystemMonitorView.swift	Two 18×18pt circular gauge rings with SF Symbol icons, ScaleButtonStyle, click-to-show popover (SystemMonitorMenuView), reusable GaugeRingView subview, color extension.
boringNotch/components/Settings/Views/SystemMonitorSettingsView.swift	Settings page with enable/disable toggle and live CPU/memory stats preview.

Modified Files

File	Change
Constants.swift	Added showSystemMonitor Defaults key (Bool, default: false)
BoringHeader.swift	Conditionally show SystemMonitorView() before mirror button
SettingsView.swift	Added sidebar link + detail routing for System Monitor settings
BoringViewModel.swift	Added isSystemMonitorPopoverActive property + reset in close()
ContentView.swift	Added popover flag to 3 notch-close guards + .onChange observer
project.pbxproj	Added PBXFileReference, PBXBuildFile, PBXGroup, PBXSourcesBuildPhase entries
Localizable.xcstrings	Added localization entries for System Monitor strings

Design Decisions

• Mach APIs only — uses host_statistics / host_statistics64 directly, no dependencies on sysctl or third-party libs
• CPU calculation — delta-based tick comparison between polls (user+system+nice vs total), not instantaneous snapshot
• Memory calculation — (active + wired + compressed) / total physical matches Activity Monitor's "Memory Used"
• Color thresholds — CPU: green <50%, yellow 50–80%, red >80%; Memory: green <70%, yellow 70–85%, red >85%
• Hover persistence — follows the battery popover pattern (isSystemMonitorPopoverActive flag prevents notch auto-close)
• Resource management — mach_host_self() port cached and deallocated in deinit to prevent Mach port leak

Testing

• Gauges display correct CPU/memory percentages (compare with Activity Monitor)
• Color transitions work at threshold boundaries
• Popover opens/closes correctly, notch stays open while popover is active
• Feature is disabled by default, toggle in Settings works
• Timer starts/stops properly when feature is toggled on/off
• No memory leaks or Mach port leaks (check with Instruments)