[codex] Polish settings behavior and app icon

[ysyyork]

Summary

• Wire existing settings so menu bar visibility, network speed units, CPU dropdown sections, and disk dropdown sections are reflected in the UI.
• Add a rescue control menu that only appears when all monitor menu bar items are hidden, preventing lockout without taking normal menu bar space.
• Replace the app icon PNG set with a refreshed macOS-style system monitor icon.
• Add formatter coverage for fixed network speed units.

Impact

This makes several existing settings behave as users expect, reduces the risk of blank or unrecoverable menu bar state, and improves the app's visual polish.

Validation

• xcodebuild build -scheme MacStatusBar -destination 'platform=macOS'
• xcodebuild build-for-testing -scheme MacStatusBar -destination 'platform=macOS'

Note: full xcodebuild test previously compiled but hung while launching the app-hosted status bar process, so build-for-testing was used to verify the test bundle compiles.