feat: allow hiding the menu bar icon

[hcaiano]

Summary

• add a General setting to show or hide Cycler in the menu bar
• persist the preference with backward-compatible defaults
• reopen Settings when a hidden Cycler is launched or reactivated, while keeping background login launches silent

Test plan

• swift build
• swift run cycler-tests — ok, 106 checks passed
• build and codesign dist/Cycler.app
• live toggle off/on/off in the General tab
• verify cold launch and running-app reopen both show Settings while hidden
• verify background hidden launch remains a non-frontmost UIElement with no visible window