[FEATURE] System Monitor — CPU & Memory usage gauges in notch header

[teszerrakt]

Is your feature request related to a problem?

Power users and developers frequently need to monitor system resource usage. Currently this requires opening Activity Monitor or a third-party menu bar app, which takes up menu bar space and adds context-switching overhead. Since boring.notch already occupies the notch area and displays system info like battery, it's a natural place to surface lightweight system stats.

Describe the solution you'd like

Add optional CPU and memory usage circular gauge rings (18×18pt each) in the notch header area (right side, before existing buttons). Features:

• Circular gauge rings with SF Symbol icons centered inside, color-coded by load:
  • CPU: Green < 50%, Yellow 50–80%, Red > 80%
  • Memory: Green < 70%, Yellow 70–85%, Red > 85%
• Click to show popover with detailed stats (CPU %, memory %, memory in GB) and an "Open Activity Monitor" button
• Polling every 3 seconds using macOS Mach kernel APIs (no new dependencies)
• Disabled by default — user opts in via Settings > System Monitor toggle
• Follows existing patterns (battery popover, settings layout, hover persistence)

Use cases & user scenarios

• Developers monitoring builds/compiles who want at-a-glance CPU load without switching apps
• Users keeping an eye on memory pressure during heavy multitasking
• Anyone who wants lightweight system health monitoring without extra menu bar clutter or third-party apps

Additional context & screenshots

I have a working implementation ready based on the dev branch. Will submit a PR shortly.

Willingness to contribute

• Yes, I can write the code for this feature

Pre-submission checklist

• I have searched existing issues to ensure this isn't a duplicate
• I have provided sufficient detail for the team to understand the request